The Federal Aviation Administration scrambled to fix the problem after issuing a nationwide ground-stop.

Hours later - the order was lifted - but not before causing thousands of flight delays and hundreds of cancellations.

The cause of the problem with a pilot-alerting system was unclear, but U.S. officials said they had so far found no evidence of a cyberattack.

President Joe Biden:

BIDEN: "I just spoke with Buttigieg. They don't know what the cause is, but I was on the phone with him about 10 minutes. I told them to report directly to me when they find out."

Transportation Secretary Pete Buttigieg pledged a "process to determine root causes and recommend next steps."

The outage occurred at a historically slow time for U.S. travel after the December holiday season, but airlines have said demand remains strong as travel continues to recover to near pre-pandemic levels.

For passengers from Denver to Orlando, the stoppage meant major disruptions to travel plans, though people seemed to take it in stride.

DENVER STRANDED PASSANGER, CHRIS WICKLUND:

"Basically, every single concourse that I was at was basically saying: 'Guys, according to the FAA, every single flight is ground stop and we can't really do anything. So you, guys, got to sit tight'. And then there were people that were boarding planes at like 5-6 a.m. They had to get off their plane because they were basically on the tarmac for like an hour, hour and a half. And then they came back because they couldn't go anywhere."

ORLANDO STRANDED PASSANGER, LOU MONGELLO:

"Everybody deplaned very quickly, very orderly. Even right now, I haven't seen anybody really upset at all. There's very, very short lines at each of the gates. But United I can speak for has been very, very proactive, very responsive in terms of providing meal vouchers, providing updates. "

By mid-day Wednesday - flights were slowly resuming...

But those efforts were hampered, as airlines struggled to get planes in and out of crowded gates.

In all-- More than 6,000 flights were delayed and close to 1,000 canceled according to FlightAware, with numbers still climbing.