The glitch was another in a long string of technology problems that began when it merged computer systems with Continental's in March.
United acknowledged at least 200 delayed flights. Its passenger reservation system and website stopped working for about two-and-a-half hours on Tuesday afternoon, although the precise cause wasn't known.
The outage didn't affect planes in flight.
Passengers in several United hubs reported very long lines at ticket counters. During the outage it stopped sending planes to its hubs in Newark, N.J., and San Francisco.
Alex Belo was waiting at Newark to get on a flight to Mexico City. He considered himself lucky to be behind only 100 or so people waiting to check a bag - because there were another 300 to 400 behind him.
"The line is not moving, or very slowly moving. And they're giving priority only to first class," he said.
United said it will not charge the usual change fees for passengers on affected flights who want to cancel or rebook their tickets. It apologized for the disruption.
Simon Duvall spent two hours sitting on his flight waiting for the computer problems to be resolved. People were calm but not happy, he said.
"We're on a plane, on the tarmac in Las Vegas in the middle of August. It's warm. It's uncomfortable. It's cramped," he said.
United Continental Holdings Inc. has been struggling with computer issues off and on since March, when it switched to using Continental's system for tracking passenger information. The two airlines merged in 2010.
Airlines rely on software to know who is filling the seats on its planes, and how many empty seats are available. Those computer systems make it possible to print boarding passes, too.
Rich Pearson, head of marketing at professional freelance site Elance, was stuck in Houston on his way to present at a jobs seminar at the Republican convention in Tampa, Fla.
Planes were lined up on the tarmac. "It's almost like horseback riding when they are all nose-to-butt," Pearson said. "It's like we've gone back 50 years."
"People are relatively calm," he added. "The customer service area was initially flooded. But they can't really do anything."
AP Airlines Writer Scott Mayerowitz contributed to this report.