From ABC News.

A tech outage led United Airlines to ground flights nationwide Wednesday night, and even with the issue now resolved, over 1,000 flights have been delayed and dozens canceled.

The outage stemmed from an issue with the airline’s weight and balance computer system, United said.

"Due to a technology issue, we are holding United mainline flights at their departure airports," the airline said in a statement. "We expect additional flight delays this evening as we work through this issue. Safety is our top priority, and we’ll work with our customers to get them to their destinations."