"We have a network outage & are working to get systems back online," the airline tweeted at 3 p.m. CDT, following 2 1/2 hours later with a message saying its Web site and flights were back online and the company was "in the process of resuming operations and rebooking customers."
"It's a zoo," Rick Cartwright, who was scheduled to depart Chicago's O'Hare for Dayton, Ohio, at 6:45 p.m., told the Chicago Tribune. "I fly in and out of here almost every week, and I've never seen as many people in Terminal 1 before; I've never seen it this crowded."
He said airlines employees were issuing handwritten boarding passes and luggage tags for at least an hour.
"The lines I'm looking at were probably several hundred people," he said. "Even if everything is back online, it's going to take awhile to get all of them through in time for their flights."
Karen Pride, a Chicago Department of Aviation spokeswoman, told the newspaper no other airlines were affected.
"The airline is issuing a waiver policy permitting customers on affected flights to cancel or rebook their itineraries without penalty," United said in a statement. "United apologizes for the disruption caused to travelers at affected airports and is re-accommodating customers as quickly as possible."
FlightAware.com said United's system was down from about 2:20 p.m. to 4:20 p.m. CDT and noted there likely will be "issues and cascading delays through the evening."
The computer problems, which caused the Federal Aviation Association to suspend arrivals in Newark, N.J., Houston, and San Francisco, were unrelated to Hurricane Isaac, NPR reported.
Passenger Bruce Rehburg, whose flight from Houston to Denver was delayed nearly 2 hours, told NPR the pilot was at the mercy of the computer glitch.
"The pilot told passengers that he knows the computer is safe and ready for takeoff, but the technology is locked. We are ready to go, and now in the age of computers, we can't take off," Rehburg told NPR.
2014: NFL Cheerleaders [PHOTOS]