The airlines posted on Twitter at 9:52am CST, "Our system is up. We're resuming operations for affected flights."
This is at least the third major computer outage for the Chicago-based airline since June.
"Does anyone have a Radio Shack computer or abacus to help United get their system fixed?," tweeted Lewis Franck, a motorsports writer who was flying from Newark, N.J. to Miami Thursday to cover the last race of the NASCAR season.
In a subsequent phone call with The Associated Press, Franck added: "Why is there a total system failure on a beautiful day? What happened to the backup and the backup to backup?"
United Continental Holdings Inc. spokesman Charles Hobart said the airline was aware of a computer issue affecting some of its flights and was working to resolve it.
Passengers are being told by pilots and airport agents that computers are down and they don't know when the system will come back. Some fliers have been waiting nearly 2 hours to depart.
Judd Shapiro of Nashua, N.H. said he got to the gate at Logan Airport in Boston and agents told him and other frustrated fliers that planes could land but not take off.
"JetBlue is taking off, American is taking off, but United is on the ground," he said. "I was having a flawless airport experience until I got to the gate."
United has been struggling with technology problems since March, when it switched to a passenger information computer system that was previously used by Continental. United and Continental merged in 2010. That system, called "Shares," has needed extensive reworking since March to make it easier for workers to use.
Michael Silverstein, who works in finance, was supposed to be on a 6:01 a.m. flight from Los Angeles to San Francisco. The computer outage had already caused him to miss one meeting and he was worried about missing another. So he walked off the plane and bought a $195 last-second ticket on a Southwest Airlines flight to Oakland, Calif.
"I'm frustrated because I'm missing a meeting that I thought I had plenty of time for," Silverstein said.
The Associated Press contributed to this report.