Delta Airlines grounded all its flights for several hours Monday because of a system-wide computer breakdown, making for a messy day at airports around the world.
Even after the US carrier lifted the order about six hours into the snafu, it warned of continued cancellations and delays, as tens of thousands of passengers around the globe were left stranded.
Travellers crowded ticket counters, lay on the ground in airports and tried to grin and bear it. Ticket agents were reduced to checking people in manually, news reports said.
The carrier blamed the computer problem on a power outage in its hub in Atlanta, Georgia.
Flights have resumed but on a limited basis, Delta said in a statement after flights had been grounded for about six hours. However, the ripple effect of the meltdown will drag on.
"Customers heading to the airport should expect delays and cancellations," the statement said.
"While inquiries are high and wait times are long, our customer service agents are doing everything they can to assist," it added.
It also said there would be "some lag time in the display of accurate flight status at delta.com, the Fly Delta App and from Delta representatives on the phone and in airport."
Luciano Resende, 40, waiting at London's Heathrow Airport to fly to San Francisco, said it was slow going.
"I guess it has been a long time since they used the manual process," Resende told NBC News.
At Los Angeles International Airport, people waiting for red-eye flights to the east coast dozed in a crowded waiting area, many of them wrapped in red blankets.
As of about 1400 GMT, the airline said it had cancelled about 300 flights. It operates more than 15,000 a day along with its alliance partners.
Nearly 400 others were delayed, according to flight-tracking service FlightAware.
"The timing of this problem is particularly bad because Monday morning is one of the busiest times for both airlines and travellers as business travellers begin their work week," said Daniel Baker, FlightAware's CEO.
Delta said the power outage in Atlanta that caused the computer meltdown began at 2:30 am (0630 GMT).
Lines of passengers backed up at Delta ticket counters at US airports.
At Los Angeles, passengers on a flight to New York had to get off their plane and return to the terminal, NBC News reported, while some people slept near departure gates at Las Vegas.
As compensation for the inconvenience, Delta offered refunds to travelers whose flights are cancelled or significantly delayed. People on some routes are being allowed to make a one-time change to their travel plans.
A vast number of flight delays normally creates a cascading problem that affects airline traffic for days.
Computer outages halting flights are not uncommon.
In May, a glitch affecting Sweden's civil aviation authority radar site disrupted air traffic throughout that country and grounded flights to and from Stockholm for several hours.
In March, a computer system malfunction forced Japan's All Nippon Airways to cancel more than 100 domestic flights, affecting some 16,000 travellers.
And in mid-August 2015, a computer problem at a regional air traffic control center delayed hundreds of flights at busy US east coast airports -- including those in the Washington and New York areas -- for several hours.
A rival US airline, United, suffered computer glitches in May and July 2015 that temporarily grounded hundreds of flights and backed up thousands of passengers.