It is thought a fire may have caused the outage
Hundreds of flights were cancelled after the busiest airport in the world lost power.
Hartsfield-Jackson Atlanta International Airport (ATL) went dark for several hours on Sunday.
The US Federal Aviation Administration issued a ground stop for all flights headed to the airport, and many flights inbound to Atlanta were diverted.
Passengers who were due to depart the airport on Sunday were all de-planed.
Georgia Power believes the issue may have involved a fire, which caused extensive damage to an underground electrical facility.
"This event impacted not only the underground facilities, but also substations serving the airport," the airport said in a statement.
The fire was safely put out by fire crews before assessments and repairs could begin.
The cause of the incident is unknown.
US carrier Delta cancelled about 1,000 flights on Sunday as it worked to recover from the power outage that shut down the airport for much of the day.
It says the power outage impacted all airlines operating in Atlanta.
Power has since been been restored.
Power is back ON in Atrium and Concourses T, A and B!— Atlanta Airport (@ATLairport) December 18, 2017
The airport is reminding passengers that those with a ticket dated December 12th will need to re-print it before being processed through checkpoints.
According to figures from the Airports Council International (ACI), the airport retained the top spot as the world's busiest.
It handled more than 104 million passengers in 2016, 2.6% more than 2015.