Major airlines have restored some operations after grounding flights across the US, citing communication issues caused by a global IT crash.
The failure – which also affected banks and emergency services – was caused by a faulty software update from cybersecurity firm Crowdstrike.
Delta Airlines and American Airlines grounded all flights on Friday but have since resumed services.
Crowdstrike apologised and said that the issue had been identified and would be fixed, although it did not say when.
More than 42,000 flights were delayed and over 4,700 cancelled worldwide, according to FlightAware.
American Airlines and Delta have told passengers they would be issuing travel waivers for the disrupted services.
US tourist Stephanie Thompson was unable to board her flight back to Dallas after a holiday in the UK.
“We couldn’t get an answer from anybody,” she said, speaking from Edinburgh Airport. “I was on hold with American (Airlines) for about an hour and 10 minutes before I finally hung up.
“We just paid $6,800 (£5,260) for a one-way trip home, hopefully leaving tonight. I didn’t know what else to do. I just wanted something to get us home.”
Los Angeles International Airport told the BBC that “some flights” were taking off and landing”, indicating that the issue was primarily affecting airlines rather than airports.
United Airlines and a number of smaller carriers were also affected.
In a statement, United said several different systems had been affected, including those that calculate aeroplane weight, phone systems and self check-in kiosks.
The first indication of the Crowdstrike issue affecting US airlines emerged late on Thursday, when US airline Frontier said a major “technical outage” had affected its operations.
Pete Buttigieg, the US transport secretary, said his department was “monitoring technical issues” at Frontier that were “leading to cancellations and delays across their network.”
The disruptions also affected hospitals, courts and government offices.
The New York-based Kaleida Health network, which runs five hospitals in the Buffalo area, said on Friday morning that its systems were momentarily down.
At Mass Brigham General hospitals, in Boston, Massachusetts, all appointments considered non-urgent were cancelled on Friday.
Vehicles travelling across both the northern border with Canada and the southern border with Mexico were also delayed.