Delta Air Lines said it canceled about 80 flights Monday — possibly more — due to an outage affecting essential computer systems.

The systems outage began at . All systems were restored to normal shortly after midnight Monday.

The hours-long outage also caused at least 150 flight cancellations for Sunday. Delta has issued a flight change fee waiver for customers affected by the outage on either day. Unaccompanied minors will not be accepted for flights until after noon Monday.

“I want to apologize to all of our customers who have been impacted by this frustrating situation,” Delta CEO Ed Bastian said in a statement. “This type of disruption is not acceptable to the Delta family, which prides itself on reliability and customer service. I also want to thank our employees who are working tirelessly to accommodate our customers.”