Atlanta (CNN) There were people still sleeping on the floor the day after a power outage shut down operations at the Atlanta airport, but long ticket and security lines were moving slowly as normalcy began returning to the world's busiest airport Monday.

Mondays and the holiday breaks are busy times for Hartsfield-Jackson International in general. Frequent fliers said the airport felt a little busier than usual, especially in the terminal that hosts Delta, which is headquartered in Atlanta.

The line for Delta's Sky Priority passengers zig-zagged back and forth seven times and was still backed up to the main entrance to the airport. Some passengers muttered, "Oh my," upon encountering the lines. Others' reactions were more profane.

Passengers wait to learn if their flights will leave Atlanta on Monday.

Volunteers in shirts that said, "Ask Me," tried to allay concerns and passed out doughnuts to those in line, many of whom shared horror stories about the night before.

While some fortunate passengers were able to board the flights departing Atlanta, other passengers were being told they'd have to wait hours or days. One airline was telling passengers it would be five days before they could get a flight out.

'They did pretty well'

When Ashley Hanford, 32, an Atlanta native who now calls Washington home, arrived for her 3 p.m. flight Sunday, the power was already out. Her flight was among almost 1,200 that were canceled Sunday.

She slept in the terminal all night. Speaking to CNN as she charged her phone around 5 a.m. Monday, Hanford said airline and airport employees did their best to take care of stranded passengers, handing out blankets, beverages, even slices of pizza.

"For it to be so chaotic, they did pretty well," she said.

Hanford planned to hang out at a cousin's house Monday before returning for her rescheduled 9:30 p.m. flight, she said.

While airline delays tend to spark short fuses, passengers seemed particularly magnanimous Monday morning. April Hubbard, who was in the security line for her flight to Houston when the airport went dark, reported a lack of communication, widespread rumors, the strong smell of fire near baggage claim and a taxi line that amounted to "pandemonium."

Stranded passengers congregate around baggage claim Sunday.

But the people stuck in the airport were "remarkably good," she said.

Another passenger, Suzi Harrington, could miss an adventure of a lifetime if she doesn't get a flight out Monday. She and three friends were scheduled to travel to Tanzania in hopes of summiting Mt. Kilimanjaro on Christmas, she said. As she stood in a long Delta ticketing queue, she said she wasn't angry.

"I mean, what are you going to do?" she said.

'Possibly millions of people disrupted'

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Electricity returned to the mammoth airport late Sunday.

Atlanta's Hartsfield-Jackson Airport: By the numbers Hartsfield-Jackson has been the world's busiest airport since 1998

An average of 275,000 passengers pass through each day

Airport handles 2,500 arrivals and departures daily, on average

Hartsfield-Jackson airport serves 150 destinations in the US, and over 75 destinations in 50 countries worldwide

Home to North America's tallest air traffic control tower (398 feet; 121 meters) -- it's also the world's fourth highest

Georgia's largest employer, with more than 63,000 employees

The lights flickered on shortly before midnight, after an exhausting day for travelers. Thousands were stranded in dark terminals and on planes sitting on the tarmac.

The blackout led the Federal Aviation Administration to declare a ground stop at the airport, preventing Atlanta-bound flights in other airports from taking off and causing inbound flights to be diverted. The ground stop in Atlanta disrupted air travel across the United States.

Shortly after power came back, some passengers lined up at security screening, hoping to beat the crowds as the TSA checkpoints reopened at 3:30 a.m.

The outage, which affected all airport operations, started with a fire in a Georgia Power underground electrical facility, Mayor Kasim Reed said.

The electrical fire's intensity damaged two substations serving the airport, including the airport's "redundant system" that should have provided backup power, Reed said.

An estimated 30,000 people were affected by the power outage, Reed said.

A traveler sleeps on a baggage carousel at Hartfield-Jackson Atlanta International Airport on Sunday.

The disruption led to 1,180 flight cancellations to and from the airport Sunday, according to flight tracking service FlightAware. As Atlanta is the heart of the US air transport system, it also caused problems at other airports, said Desmond Ross, principal of DRA Professional Aviation Services.

"We're talking possibly millions of people disrupted over the next few days and it is certainly not going to be fixed in one day," Ross said.

Officials: Switchgear box failure caused fire

Passengers wait in a dark terminal at the Atlanta airport Sunday.

At 12:38 p.m., Georgia Power noticed outages in the system that were traced to a fire in underground tunnels where the airport's electric system lives, spokeswoman Bentina Terry said. The fire caused multiple faults that led to the blackout at 1:06 p.m.

The fire's intensity prevented Georgia Power crews from immediately accessing the tunnels to restore power, the mayor said. By 3:30 p.m., fire crews contained the fire.

"We know there was a failure in the switchgear that caused the fire that absolutely created the issue," Paul Bowers, Georgia Power's president and CEO, told ABC's "Good Morning America." A switchgear is a collection of devices that helps control power distribution.

When the switchgear ignited beneath a tunnel housing Georgia Power's cables, he said, it impacted cables in both directions and burned a wall through which the cables travel, taking out both the main system and the backup system.

Investigators were in the tunnel Monday morning, trying to answer several questions -- including whether the fire was deliberately set -- and workers will have the cables fixed this week, Ross told the morning show.

The failed switchgear box was located under Concourse C, in an area that is restricted by key-card access only and monitored by video surveillance, said Sgt. Cortez R. Stafford, an Atlanta Fire Rescue spokesman. Stafford said there was no unauthorized entry before the fire.

Authorities concluded their investigation and determined there was no evidence of foul play, according to Stafford.

Stuck for hours

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After the airport went dark, people used their phones to light their way through shadowy terminals. Nothing worked. The train between terminals was shut down. Elevators, escalators, automatic doors and baggage carousels stood still. Screens were black. The intercom that normally squawks flight updates was silent.

No one could get reliable phone or internet service to access texts, email, flight apps or social media.

Meanwhile, amid the dearth of information, people were too nervous to leave their spots, fearing the power might soon return at any moment and they'd lose their place in whatever line they were in.

The bathrooms quickly turned malodorous. The sensors on toilets, sinks and soap and towel dispensers stopped functioning, leaving people futilely waving their hands in front of them. Custodial staff put out giant rolls of paper towels.

Shayne Plemons of Palm Beach, Florida, and her husband were headed home and had a layover in Atlanta that landed just minutes after the blackout. They stayed on the tarmac for six hours. The plane had power and the pilots kept passengers informed about developments -- or the lack thereof -- but the plane ran out of food after about three hours and the crew at one point asked passengers to stop flushing toilets, Plemons said.

There was plenty of water and other drinks, though, she said. Plemons ended up watching movies until passengers were able to disembark onto the tarmac.

"Under the circumstances, it was well-managed. (Delta employees) were doing the best they could," she said. "They kept everyone fed, watered and comfortable."

Joe Ryan, 27, had less luck staying fed. He arrived about 5:30 p.m. for his and his fiancee's flight to Chicago and found the airport dark.

"I'd heard it had been out for a while, but I thought, 'Man, they must be closer to getting it on,'" he said.

Finding no restaurants open, Ryan ordered a pizza to be delivered to a nearby MARTA station in College Park and took the train to go pick it up and bring it back to the airport's second-floor atrium, where he and his fiancee spent the night. On Monday morning, both Ryan and the Plemonses were still waiting, hoping to catch planes home in the afternoon.

If they didn't make those flights, both couples were told it might be Wednesday before they could fly home.