Passengers say the issue started around 8 a.m. Central Time, affecting 250 United flights at airports across the country, according to an airline spokesperson.
At O'Hare, the delays piled up as United planes were left sitting near the runway waiting to take off.
"Really I think what set people off a little bit was that we, at one point, were told we don't know if we're going to be leaving. We'll have to see if the computers come back up," said Richard Price, United passenger.
"I think the captain was quite forthright. He explained about the computers," said Judita Cuplinskas, United passenger.
After a nearly three-hour wait at O'Hare, Nancy Ellen-Zusman decided to call it a day.
"And they let some people get off the plane but stay in the boarding area. Then we can get back on the plane, and they told us we were ready to go. And then they announced there was a maintenance problem," she said.
At 10 a.m., about two hours after the problems started, United said its computers were back up, allowing flights to resume.
"We did find it a little funny, though, that the little video debriefing that you get on safety started off with a great advertisement for their new computer system. So everyone got sort of a chuckle about that," said Price.
"It definitely put a dent in my day. Missed a couple meetings, but hopefully I'll be able to make them up," said Tess Beeler, United passenger.
Thursday's outage marks at least the third time since June that a United computer glitch has caused widespread delays. The problems seem to stem from United's adoption of Continental's computer system following the merger of the two airlines.
"The merger issue is related to technology and I think there's both questions about this new share system and why the management team didn't have a workable backup," said Joe Schwieterman, DePaul University professor.
Last June, United gave ABC 7 a tour of its new high-tech operations center in Willis Tower, which, according to the airline, is allowing for a more streamlined operation post-merger.
"Unfortunately this is just something that happens. But hopefully on Tuesday when I'm flying back in more rush of the Thanksgiving traffic that this won't happen again," said Beeler.
Statement from United:
For about two hours this morning, an outage in one of our flight operations systems caused the delay of about 250 mainline United flights. We found and corrected the error, and by mid-afternoon, systemwide on-time performance was approximately 80 percent. We apologize for the inconvenience this caused our customers. We are offering flexible flight-change and refund options for those customers affected today.
The Associated Press contributed to this report.