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 Air Group resumed flights after a data center hardware failure triggered a major IT outage, grounding its entire fleet for hours. The issue caused over 150 flight cancellations and disrupted operations across Alaska Airlines and Horizon Air.

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.