UPDATED: 5:14 PM ON 8-8-16
Delta says its systems are now fully operational, but delays and cancellations will continue as the company recovers from the power outage.
Delta says some critical systems and equipment failed to switch over to backup systems after the outage, and it is investigating why that happened.
Affected customers will get vouchers for new flights.
UPDATED: 9:37 am on 8-8-16
A ground stop for Delta flights has been lifted, and limited departures are resuming, the airline says.
The airline’s flights were grounded for at least six hours, causing large-scale cancellations and stranding hundreds of thousands of passengers.
Delta, the world's second largest airline, said the problem was a power outage at its Atlanta hub.
"Customers heading to the airport should expect delays and cancellations," Delta warned.
"Delta has experienced a computer outage that has affected flights scheduled for this morning," the company tweeted to customers affected by delays. "Flights awaiting departure are currently delayed. Flights enroute are operating normally. We appreciate your patience."
The company said its IT systems were down everywhere.