CHICAGO – United Airlines passengers across the country were stranded for at least five hours Friday following a computer crash that grounded flights.

United Airlines announced to passengers in Denver that it was preparing to resume service shortly after 10 p.m. PDT – about five hours after the computer problems started.

The airline did not immediately return calls seeking comment about whether flights were resuming everywhere.

Earlier, the airline confirmed the problem on Twitter. United spokesman Charles Hobart did not elaborate on the nature of the problem. He said the airline is apologizing to travelers.

As a result of the outage, long lines of passengers formed at check-in counters at Chicago’s O’Hare International and Denver International airports, two of United’s largest hubs.

Staff at Denver International Airport made the announcement on the loud speakers when computers were slowly coming online. A flight to Washington, D.C.’s Dulles International Airport was being prepared for departure as computers booted up.