DALLAS — A day after a systemwide technical outage led to more than 1,000 canceled flights, Southwest Airlines spent Thursday trying to reboot its operations. It was one of the largest network disruptions in the company’s history.

The Dallas-based company’s computer systems were back online early Thursday after about a 12-hour outage, according to company executives.

But with thousands of customers and their bags as well as flight crews stranded overnight across the country, much of Thursday was spent trying to put the pieces of its network back together.

Southwest said it canceled nearly 700 of a total 3,900 flights Wednesday and about 450 more through Thursday afternoon. Hundreds more flights were delayed.

“First of all I want to apologize to all of our customers. This is not the kind of service that we’re famous for at Southwest Airlines. … This is all on us,” CEO Gary Kelly said in an early morning interview with CNBC. “The operation just needs some time today to catch up. So it will be another tough day today, but not nearly as bad as yesterday.”

Chief Operating Officer Mike Van de Ven attributed the technical outage – which knocked Southwest’s website and many other customer-facing parts of Southwest’s operations offline, including passenger check-in, boarding passes and ticket booking – to a failed network router.

A backup system also failed, extending the outage. Ultimately the company had to replace the router and reboot 400 servers, Van de Ven said.

“We have redundant systems that should have kicked in place and they didn’t,” he said.

Wednesday’s outage translated to long lines at check-in counters and security checkpoints at airports around the country as displaced passengers tried to catch new flights Thursday.

Customers affected by Wednesday’s outage will be able to re-book their flight at no additional charge anytime in the next two weeks, the company said. They’ll also have the option to receive a refund or travel credit instead.

Kelly estimated the company missed out on up to $10 million in lost booking revenue because of the website outage. He said some of that will be made back as customers who couldn’t book Wednesday come back and buy tickets throughout the next week.

The company extended a fare sale scheduled to end Thursday by a week for customers who missed their chance to book.

“Yes, we’re worried about the financial impact of this, but what is far more important is the inconvenience that we caused our customers yesterday and today,” Kelly said.

“I feel very bad about that. We’re very apologetic and we want to just work hard to restore their confidence in us.”