Here's what you need to know:
The problem was caused by an early morning power outage in Atlanta, where the airline is based, according a statement from Delta.
"A power outage in Atlanta, which began at approximately 2:30 a.m. ET, has impacted Delta computer systems and operations worldwide, resulting in flight delays," the company said.
But Georgia Power said the issue had to do with Delta's own equipment, not a larger power outage. "We believe Delta Air Lines experienced an equipment outage; other Georgia Power customers were not affected," said John Kraft, spokesperson for the utility. Georgia Power has staff on site trying to assist Delta, he said.
"Our IT team is still looking into the root of the problem," said Catherine Sirna, a Delta spokesperson.
As of an update at 8:40 a.m., the company lifted the ground stop on all of its planes and some limited departures were resuming. By 1:30 p.m., Delta said it had canceled 451 flights and was operating 1,679 of the 6,000 scheduled flights -- but that cancellations and delays continued. Delta chief executive Ed Bastian also appeared in a video apologizing to customers.
What should I do if I'm supposed to fly on Delta today?
In a statement posted at 5:05 a.m., the company urged travelers "to check the status of their flights this morning while the issue is being addressed." But in a later update, the company said all of its flight status systems -- including those on monitors at airports -- were incorrectly showing that flights were on time.
Delta has now issued a waiver saying that all passengers on flights that are canceled or significantly delayed can get refunds. Even if passengers' flights today haven't been canceled, the company said travelers will be able to make a onetime change to their tickets without paying a fee as long as the ticket is reissued on or before Friday.
"However, a difference in fare may apply" if the passenger re-books after Friday, according to Delta.
How often does this sort of thing happen?
While the Delta outage on Monday is massive, it's not unprecedented. Other airlines have suffered computer errors or glitches that have caused chaos for customers. Just last month, Southwest Airlines suffered intermittent computer problems over several days that canceled hundreds of flights and caused delays. Last year, United Airlines had to delay its planes for almost two hours -- affecting nearly 5,000 flights -- because of a computer glitch.
Computers and automated systems have increased the efficiency and productivity of businesses in ways that were unimaginable a century, or even decades ago. But whether because of cyberattacks or just plain computer errors, the inter-connectivity built into almost all aspects of our lives means that one problem can quickly cascade into a catastrophe. So companies need to have a plan in place for when something goes wrong.
In Delta's case, the fallback seemed to be returning to pen and paper: Some airport agents started writing out boarding passes by hand, according to NBC News.
This story has been updated. An earlier version of this story identified the Delta spokesperson as Katherine Sirna, her name is spelled is Catherine Sirna. We regret the error.