American Airlines was forced to ground all flights across the US for several hours after a key computer system failed, causing thousands of passengers to be stranded at airports and on planes.
The failure on Tuesday caused delays and cancellations nationwide. En route flights continued to their destinations but planes on the ground were not allowed to take off.
The airline blamed its computerised reservation system, which is used to track passengers and bags, monitor boarded passengers, update flight schedules and gate assignments and file flight plans.
As of mid-afternoon, American and its American Eagle offshoot had cancelled more than 700 flights and another 765 flights were delayed, according to tracking service FlightAware.
The outage, which began in midmorning, was fixed by 2130 GMT, airline spokeswoman Stacey Frantz said.
"Flights have resumed, but we expect cancellations and delays throughout the remainder of the day," the carrier said in a statement.
Travellers whose flights were cancelled complained that they were not able to rebook on a later flight.
"Tensions are high. A lot of people are getting mad. I've seen several yelling at the American agents," said Julie Burch, a business-meeting speaker who was stuck at Dallas-Fort Worth International Airport waiting for a flight to Denver.
"Nobody can tell us anything.''
Terry Anzur, a TV news consultant from Los Angeles who was also stranded in Dallas, said American Airlines gate employees were operating manually because of the affected computer network.
"No one at the counter can do anything. They can't check people in," Anzur said. "The airline is at a dead halt."
American Airlines is on its way to becoming the biggest airline in the world after winning bankruptcy-court approval to merge with US Airways
The combined airline plans to use the American system that broke down on Tuesday.