Here is how the day unfolded:
7 p.m.: The outlook for other airlines is also improving.
American Airlines canceled 43 flights out of Atlanta. By Monday afternoon a spokesman said the airline was back to normal.
Experts expect unraveling all the delays and canceled flights could take three to four days.
5:30 p.m.: Delta reported Monday afternoon that operations were returning to normal. The airline began the day with 390 canceled flights as of 1 p.m. and had not added to that that, officials said in its latest update. Delta canceled about 1,400 flights over the nearly 24-hour period of the and and after the airport fire and power outage.
About 80 percent of the travelers coming through the Atlanta airport each day are Delta customers, “so we were closely engaged at all levels with the airport, City of Atlanta and Georgia Power in their recovery efforts following Sunday’s outage,” said Gil West, Delta's Senior Executive Vice President and COO.
READ | Atlanta’s kink snarls nation’s air-travel net
MORE | Atlanta’s power outage a ‘wake-up call’ for nation’s airports
5 p.m.: The large number of flight cancellations forced Kennesaw State University women’s basketball team to miss a tournament in Puerto Rico.
The team was set to fly out Sunday for the tournament that began today. Instead, the team will have some team bonding activities before Christmas break, head coach Agnus Berenato said.
4 p.m.: It's back! The SkyTrain connecting the airport to the rental car center has resumed service, according to a tweet from the airport.
The SkyTrain was not operating earlier Monday, and travelers were being taken to the rental center by shuttle bus.
Mayor Reed: ‘We’re going to fix this problem’ at the airport
3:30 p.m.: Monday afternoon Atlanta Mayor Kasim Reed said that a series of fixes at the airport would prevent another electrical fire that knocked out power like the one Sunday night. Reed, who was at the Hamilton E. Holmes MARTA Station for an unrelated event, said the airport would invest in “more aggressive portable lighting capability to large parts of the airport are not left in the dark again.
He also apologized for delays in communication from officials to stranded and confused passengers, said officials didn’t know much early on.
When will Atlanta airport be back to normal? Depends on your airline
2:10 p.m.: The SkyTrain from the airport to the rental car center is currently down, said airport spokesman Reese McCranie.
There is no estimated time for the train to restart, and the airport is using buses to get travelers to the rental car area.
The SkyTrain operates six two-car trains that can carry 100 passengers and their baggage, according to the airport's website. The train is a free service to and from the airport.
Earlier Monday, the “Plane Train” connecting the concourses was down, but was restored shortly after 10 a.m.
1:30 p.m.: Operations at the Atlanta airport are expected to be back to normal by midday today, according to the Federal Aviation Administration.
READ | Airport shutdown is a worrisome tale of vulnerability, poor time for Georgia Power
When power was restored around midnight, the FAA said it had full staffing in the Atlanta control tower to handle commercial flights when they resumed.
About 10:30 a.m. today, airport GM Roosevelt Council told WABE the airport was at 90 percent normal functioning.
1 p.m.: There’s some good news out of the airport outage. UPS and FedEx say the fire that knocked out power and shut down the world’s busiest airport won’t delay holiday package delvieries.
A FedEx spokeswoman described the firm’s operations as “absolutely normal.”
The U.S. Postal Service also reports no disruptions in its holiday delivery service.
12:30 p.m.: Also, some good news for stranded passengers: Papa John's pizza tweeted that the pizza chain will be delivering pizzas to the airport today.
On Sunday, Atlanta-based Chick-fil-A mobilized employees to prepare and deliver sandwiches to hungry airport travelers, putting aside its long practice of closing on Sundays.
According the Chick-fil-A spokesperson Jackie Jags, “The mayor called about 10 p.m. and asked for assistance. We immediately mobilized staff and team members who live and work near the airport, and they are making sandwiches and delivering them to the EOC (emergency operations center).”
City and airport officials distributed the sandwiches to the stranded passengers.
NOON UPDATE: Chris Kuczek got by with a little help from his friends. He was stuck on his plane for five hours Sunday due to the power outage, but his fellow travelers appreciated his blue-eyed carry-on — 12-week-old Australian shepherd named Juno.
“Everybody was thrilled to have her around,” he said.
Kuczek, 29, is stationed with the Air Force in Pensacola, Florida, but was trying to head back to New Jersey for the holidays when he got stuck in Atlanta.
Thankful for him and Juno, Kuczek had a friend he could crash with in the metro area.
Delta rebooked him for a flight he missed and he and Juno are set to leave Atlanta at 5 p.m. Monday.
10:37 UPDATE: You've got questions, we are working to find answers. AJC reporters are working to find out more about the fire that caused the power outage, how airlines are moving stranded passengers into and out of Hartsfield-Jackson and what kind of long-term impacts this outage could have on the airport.
If you have a question, please let us know. Here’s what we’ve answered so far:
10:30 a.m. UPDATE: Airport GM Roosevelt Council told WABE the airport is at 90 percent normal functioning. No waits at TSA lines. If you were at the airport this morning, it would 'look like any other Monday morning.' Responding to suggestions that airport seemed unprepared, he sidestepped and said nothing like this had ever happened before and this event was an 'anomaly.' He blamed the failure of the backup power on a switch that was affected by the fire. He said it was unclear if switch itself was affected by the fire, or if it was shut down as part of the response. Said it was not accurate to say the airport did not have redundant power.
10:20 a.m. UPDATE: While airport power was restored about midnight, the "Plane Train" connecting the concourse was not operating normally Monday morning. Passengers were largely forced to walk the long underground pedestrian tunnels between concourses. But shortly after 10 a.m., airport spokesman Reese McCranie said the train was back in service.
9:17 A.M. UPDATE: It wasn't just out-of-towners with connecting flights who were held up by the outage.Bridget Maxwell, 53, of Powder Springs, was in the airport atrium waiting for her son to finally arrive after his flight was held overnight in Greensboro, North Carolina.The Boston University freshman left to head back home to Cobb County at 1 a.m. from Boston. The family wonders why his plane was allowed to leave with the power off at his destination.He managed to forage beef jerky and water from the sleepy Greensboro airport overnight, his mother said.He landed in Atlanta about 9 a.m. and his mother expects him to sleep for a good long while.
8:00 A.M. UPDATE:The massive outage didn't just leave passengers stranded overnight Sunday, it also affected travelers with flight Monday morning.
Kara Bowling, who lives in south Georgia and was traveling with her 16-month-old daughter and sister-in-law, arrived at Hartsfield-Jackson to find a terminal filled with passengers and lines that snaked through the baggage claim area.
Their flight to Indianapolis was scheduled to depart on-time.“The flight is taking off,” Bowling said, “we just won’t be on it.” Bowling said she knew there would lines and long waits, but they didn’t anticipate such a crush of travelers. They waited in one line to print boarding passes, but were told by one agent they had to get in another line to print a boarding pass for her baby.
She said the agent cited a computer issue. Bowling’s sister-in-law, Elisabeth Bowling, said a Delta representative told her they would be able to change flights without charge.
7:43 A.M. UPDATE: Paul Bowers: "From our standpoint, we apologize for the inconvenience." Paul Bowers, the president and CEO of Georgia Power, told The Atlanta Journal-Constitution.The utility restored power to the airport shortly before midnight.
Crews are monitoring the fixes that restored power and investigating what caused the fire and why it was able to damage redundant systems.He said the fire occurred in a tunnel that runs along the path of the underground Plane Train tunnel near Concourse E.
Sixteen highly trained personnel worked in the passageway to reconnect the network.“Our investigation is going through the process of what do we do to ensure we have the redundancy going back at the airport, because right now we are a single source feed,” Bowers said.
“We will have that complete by the end of the week, and then we will turn to what caused the failure of the switch gear.”
Though the cause isn’t yet known, he said foul play is not suspected.“There are two things that could happen,” he said.
“There are inner workings of the switch gear that could create the heat that caused the fire, or the splicing going into that switch gear -- that the cable had a failure on that going into the switch gear.”
When asked if age of the system could have been a failure, Bowers said his company conducts regular inspections.“We constantly inspect,” he said. “We inspect on an annual basis to ensure the reliability of the network, and that redundancy is protection for the airport.”Bowers said he is not familiar with any similar fire or outage at the airport.
“The issue for us is to ensure the reliability is here and that it doesn’t happen again and to ensure that our network is resilient enough to withstand any kind of fire,” he said.
7:26 A.M. UPDATE: Phito Moleus, a Boston attorney, was in Atlanta for a conference and arrived at the airport for a 4:45 p.m. return flight."We came in and - chaos," he said. "No information. We went to one line and were told we had to go to another."
No one wearing a uniform seemed to know what was going on or where passengers should go, Moleus said. By 7 p.m., he’d found a hotel. But he was back in line early Monday. He still didn’t know when he’d be able to catch a flight.
“We would expect this from a third world country,” Moleus said. “I do hope they learn from this.”
7:05 A.M. UPDATE: Bowers says Georgia Power will seek to determine what can be done in the future to avoid a similar event.
7:04 A.M. UPDATE: "We're going through a process of investigating exactly what happened," Georgia Power CEO Paul Bowers said. There was a failure in the switchgear that caused the fire, he said. Officials do not know if the fire was set intentionally, Bowers said.
7:03 A.M. UPDATE: Olympian Lolo Jones is among the frustrated passengers, according to Good Morning America. The outage is the lead story on the morning program.
6:39 A.M. UPDATE: Screens that typically show the status of incoming and outgoing flights at Hartsfield-Jackson International Airport are blank.
6:33 A.M. UPDATE: "Yesterday was about getting power back online," Georgia Power spokesman Jacob Hawkins said. "Now we are trying to determine cause."
Officials believe a fire in an underground power facility that damaged an adjacent switch to redundant systems caused the widespread outage.
6:30 A.M. UPDATE: Georgia Power CEO Paul Bowers will be on Good Morning America at 7 a.m.
6:26 A.M. UPDATE: Steve Grado was in the rotunda surrounded by passed-out people and three free bottles of water. Unlike others, he was awake and knew when his flight out of town would be.
He landed at 12:50 a.m., but was stuck on his Delta plane for three hours before making it off because there was no power to get the jetway to the plane door. The pilot told them at first they were staying aboard because there was no power in Concourse B due to a construction accident.
Grado, heading back home where he teaches at forestry Mississippi State University from Pittsburgh, remembers disembarking and swapping theories with folks he couldn't see in the dark concourse.
The 68-year-old lugged his two bags to the rotunda.
"It was a little daunting there for a little," he said. " ... It was pretty exhausting."
By the time he got onto the airport's WiFi, Delta had automatically booked him onto a 9:14 a.m. flight.
6:14 A.M. UPDATE: Selina Ausmus landed in Atlanta about 11 a.m. on Sunday. Her connecting flight from Birmingham was right on time and she waited for her final leg to San Jose, Calif.
“I was on my way to spend Christmas with my family,” she said. Then the lights went out.
She said airport staff handed out water and pretzels a few hours after the outage. But stranded passengers were uncomfortable. Water fountains weren’t working, she said, and many travelers didn’t eat for hours.
But what travelers really wanted, she said, was information. She said she learned far more from relatives watching the event unfold on CNN and social media than from airport staff.
“We had no clue what was going on,” said Sue Collins of Hilton Head, who was also in route to San Jose. “We were just sitting in the dark.”
Ausmus and Collins, like others, found a flat spot on a baggage carousel for the night. Ausmus and Collins also said they were fortunate to be booked on seats for a Monday morning flight.
Staff writers Eric Stirgus, Greg Bluestein, Mitchell Northam and Tia Mitchell contributed to this report.
Stay with The Atlanta Journal-Constitution for the latest updates.