United Airlines said it was working to get domestic flights on the move again after a nationwide ground stop Sunday because of a computer problem.
The ground stop, which lasted about 2 ½ hours was lifted shortly after 8 p.m. CST.
"UPDATE: The ground stop has been lifted. We’re working to get flights on their way. We apologize for the inconvenience to our customers," the airline tweeted.
United initially announced the ground stop shortly after 7 p.m. CST, tweeting amid dozens of replies to passengers' complaints that flights were grounded "due to an IT issue."
"A ground stop is in place for domestic flights due to an IT issue. We’re working on a resolution. We apologize for the inconvenience," the tweet said.
Sixty-six flights were canceled at O'Hare Airport by 8 p.m. CST, according to the Chicago Department of Aviation. Delays were averaging around 22 minutes.
It was not immediately clear how many flights in total were affected.
Chicago-based United Airlines and United Express operate more than 4,500 flights a day to 339 airports across five continents.