The airline announced on Twitter shortly after 1 a.m. CDT Saturday that its computer systems were up and running, about five hours after the systems went down. The airline said it was in the process of resuming operations. Some passengers had to stay the night in the cities where they had been stranded or were waiting in terminals filled with people wanting to get on flights.
United did not say what caused the computer issue or how many passengers or flights were affected. The airline did not return calls and emails seeking comment.
As a result of the outage, long lines of passengers formed at airports in San Francisco, Los Angeles, Denver and Chicago.
The plans of landscape designer Stephanie Hochman, 26, of Denver, to fly to Wichita, Kan., to visit her family were stymied.
"I was rushing, because I was running a little late," she said. "I kept checking computers to see if the flight was still on time. I thought it was all good, until I got to the airport and saw the people standing around at the checkout counter."
Later, staff at Denver International Airport made an announcement over the loudspeakers, saying computers were slowly coming online. A flight to Washington, D.C.'s Dulles International Airport was being prepared for departure.
Some passengers had to spend the night where they were stranded.
Nina and Mark Whitford of Brockville, Ontario, ended up in Chicago while on a layover on their flight home from Minneapolis. They said they were headed to a hotel to spend the night and were dismayed when an airline worker told them they would have to mail in their hotel receipt to get reimbursed.
"We've been waiting here for about two hours for our baggage, and nothing's come," said Nina Whitford, 35.
She said several people were still at the airport around 1 a.m. CDT Saturday, and others on their flight had rented cars to complete their trip to Canada.
"Some people were sleeping and some people were getting very angry because no one was giving us any answers," she said.
At United's terminal at San Francisco Airport, well over 1,000 people were standing around as lines slowly began to move.
Ed Costa, 62, was headed home to New York City. He said he's diabetic and had been in line almost seven hours.
"Why don't they have the backup for the system?" he asked.
Others took the delays in stride.
Pippa Davis, 50, of Christchurch, New Zealand, was on her way to Manchester, N.H., with her 11-year-old daughter, Fritha. She said they recently had earthquakes at home, including one that damaged her house.
"I think this is kind of funny, really," she said. "We're in line, but it's not for food or water."
"It's OK. We'll survive," Davis added.
Kristen Lovullo, 29, was at the Los Angeles International Airport waiting for a flight to Buffalo, N.Y. She said United employees were passing out water to passengers. She said the mood was mixed.
"Some people are complaining," she said. "Some people said they've standing outside since 5 p.m. (PDT) and they have to go to the bathroom and they're hungry."
Associated Press writers John S. Marshall in San Francisco and Denise Petski in Los Angeles and photographers Rick Bower in Denver and Charles Arbogast in Chicago contributed to this report.