Hundreds of Alaska Airlines flights and thousands of passengers’ travel plans were disrupted over the weekend and well into Monday following a “significant IT outage” that resulted in the grounding of planes for about three hours.

Alaska Airlines warned of more flight disruptions on Monday after an unexpected failure of a critical piece of hardware at its data centers late Sunday forced it to halt all flights for about three hours.

Alaska Airlines says it has resumed operations early Monday morning, July 21, after an IT outage caused the airline to issue a temporary ground stop.