The problem started shortly after 5 a.m. and at one point, the airline instituted a "ground stop," keeping United flights bound for O'Hare grounded if they hadn't taken off yet, said Federal Aviation Administration spokeswoman Elizabeth Isham Cory.
She said the glitch affected the boarding pass system and the computers that tell pilots the number of passengers on board and weight of the plane, which affect how much fuel an aircraft can carry.
"You need that to take off," Isham Cory said.
The ground stop order was lifted shortly before 8 a.m., when the airline began resolving its computer issues, she said.
By 10 a.m., at least some of the computers used to check in passengers were working.
Because the outage involved United's ticketing system, the airline urged customers to monitor their flight status and check-in online before they got to O'Hare, Urbanski said. United employees manually processed customers checking in, and self check-in kiosks were closed, she said.
At the peak of the problem, at least 1,000 passengers stood in slow-moving lines in the O'Hare terminal, and planes lined up on the airport's tarmac as new flights arrived and departing flights were held back.
O'Hare is a United hub, and the problems caused delays at other U.S. airports.
Lois Norder told the Chicago Tribune that the pilot on her flight to Dallas announced that they couldn't take off because the computer problems were keeping crews from refueling the plane.
"The employees are really stumped," Norder said. "They're trying to figure out what to do."
Shahzil Amin said the computer problems caused his flight from Dallas to Orlando via Denver to be delayed for three hours, so the airline rerouted him instead through Chicago.
"I've had a lot of problems today," Amin said.
"I pretty much won't fly United (again)," he said.
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AP Airlines Writer Joshua Freed in Minneapolis contributed to this report.
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