LaGrue said investigators found evidence of phishing attempts and ransomware. She added no city employees provided information to would-be scammers.
The City of New Orleans' Twitter account tweeted a copy of the declaration of state of emergency.
Information remains scarce. Cantrell said no ransom demand has been made.
The incident is being investigated by city officials, Louisiana State Police, the Louisiana National Guard, the FBI and the Secret Service.
It is not clear when the systems will be back online or who was behind the attack. The New Orleans cyberattack came days after a cyberattack downed computers in Pensacola, Florida, after an attack at its naval base.