Families with young children stranded at the airport for Christmas. Flight attendants and pilots sleeping on floors. Vast piles of luggage — some with presents inside, some with medication — stuck in the wrong airport. And frustrated travelers stuck on hold for hour after hour.

Southwest Airline's cascading failures have checked a veritable bingo card of travel nightmares. And while every airline faced bad weather and cancellations last week, only Southwest fell apart.

Southwest now says operations have returned to normal. But what happened? What's next? The company still has a lot of explaining to do, but here's what we know so far:

It wasn't just the weather — outdated systems helped cause the crisis

A massive winter storm caused the initial flight disruptions, but it was the company's internal software systems that seem to have turned a normal problem into an astonishing disaster.

Many airlines use a "hub and spoke" system, routing flights through a few big airports to cut costs. Southwest has long prided itself on using a "point to point" system instead. It's a leaner system day-to-day but also means lots of complex scheduling challenges to get planes, pilots and flight crews in the right place at the right time.

By all accounts Southwest was using badly outdated computer systems to manage that complicated system.

Southwest Airlines CEO Bob Jordan compared the airline's debacle to a "giant puzzle" that needs to be solved. And he said the company clearly needed to accelerate its "already existing plans to upgrade systems."

The Department of Transportation says it is launching its own investigation into exactly what went wrong.

The epic failure of the airline caught many by surprise

Southwest isn't a fly-by-night operation, or a bare-bones discount airline where customers have low expectations and misery is part of the bargain. It was a well-respected — in some cases, even beloved — company.

"They've got the best reputation for customer service and management agility," airline analyst Richard Aboulafia told NPR. "They're usually pretty good at responding to crises."

Customers are bewildered by how terrible this experience has been.

"I have 50,000 miles with them," said Hillary Chang, a traveler whose bag is lost in the Southwest disaster vortex. Now, she says, "I've been thinking about it ... I'm open to dating another airline."

Customers aren't the only ones angry. Employees are frustrated, too

The president of the union representing Southwest pilots called the Christmas meltdown "catastrophic" but told NPR he, for one, wasn't surprised by it — and neither were most pilots.

"We're still using, not only IT from the '90s, but also processes [from] when our airline was a tenth of the size," he said. "And it's really just not scaled for an operation that we have today."

There were multiple scheduling meltdowns in the last two years that, while smaller than the Christmas disaster, indicated that Southwest had a problem. Pilots were ready to work, but Southwest didn't have planes or routes available for them. The same situation unfolded in this disaster, and many pilots and flight crews took to social media to express frustration with their own company.

Customers may be reimbursed for "reasonable" expenses (... whatever that means)

Southwest is required by law to offer a full refund for a canceled flight. It has also previously committed, for any avoidable cancellation or extreme delay, to rebook passengers at no additional cost, and offer vouchers for meals and hotel accommodations.

And Southwest seems willing to cover even more costs for this debacle. But the company has not offered clear guidelines about what expenses they will cover, only saying that they will honor "reasonable requests for reimbursement for meals, hotel, and alternate transportation (such as rental cars, or tickets on other airlines)."

And of course, there's no reimbursement for missing Christmas with your family, or spending a night on an airport floor with a cranky toddler and no luggage.

Southwest has lots of apologies, and not a ton of answers

For its part, Southwest is sorry. Really sorry. The CEO is sorry. The Chief Commercial Officer is sorry. "We cannot apologize enough," customer service reps are telling furious passengers on Twitter. (They seem to be giving it their best shot, though.)

Meanwhile, the FAQs on Southwest's "Travel Disruption" site seem not so much helpful as Kafkaesque.

What should you do if receiving an error message while attempting to rebook online? "We encourage you to keep trying to rebook," Southwest advises.

What should you do if you can't find any seats on flights? "We encourage you to keep looking," Southwest says.

And what if you are stuck on hold for hours and can't get through to an agent? "If you need to reach us urgently, you may continue to call."