Delta Airlines grounded all its flights worldwide Monday because of a computer breakdown, but lifted the order hours later while warning of cancellations and delays.
The carrier blamed the computer problem on a power outage in its hub in Atlanta, Georgia.
The halt left tens of thousands of passengers around the world stranded, as they crowded ticket counters and lay on the ground in airports.
Flights will now resume on a limited basis, Delta said in a statement. But the ripple effect of the snafu 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.
Delta said the power outage 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.
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.