INTERNATIONAL NEWS - The airport is the world's busiest, handling more than 250,000 passengers and almost 2,500 flights every day.
Passengers were left in darkened terminals or on board aeroplanes.
Power is now restored at the airport, but over 1,000 flights were cancelled with hundreds of more cancellations scheduled for Monday.
Many flights scheduled to arrive from other airports were being diverted elsewhere or held at their departure airport.
In a statement, the airport confirmed they had suffered a power outage shortly after 13:00 (18:00 GMT) on Sunday. Flights are expected to resume at 06:00 on Monday (11:00 GMT).
Georgia Power, who supplies the airport's electricity, said they believed a fire at an underground electrical facility was to blame for the outage. They said a piece of its switchgear could have failed and started the fire, causing cable damage.
Power was fully restored to the airport late on Sunday.
Atlanta's mayor confirmed the fire's cause was under investigation, and apologised to the thousands affected.
A number of major airlines, including United, Southwest and American Airlines, completely suspended their operations on Sunday.
