Delta Air Lines Inc cancelled hundreds of flights and delayed many others on Monday after an outage hit its computer systems, grounding planes and stranding passengers of one of the world’s largest carriers at airports around the globe.

Atlanta-based Delta, the second-largest US airline by passenger traffic, said it had canceled 451 flights after a power outage that began around 2.30am in Atlanta.

Flights gradually resumed about six hours later.

The disruptions dealt a blow to Delta’s efforts to use its recent success in avoiding flight cancellations to win over corporate and leisure customers. The financial impact from Monday’s flight cancellations was not yet clear. Delta said passengers booked for travel August 8-12 would be entitled to a refund if their flight was canceled or significantly delayed.

The reason for the cancellations are a cause for concern and needs to be explained

The airline said late on Monday it would provide $200 in travel vouchers to all customers who experienced a delay of greater than three hours or a cancelled flight as a result of Monday’s disruptions.

Analyst Jim Corridore of S&P Global Market Intelligence downplayed the impact to Delta, saying it was “not as severe as a mild snowstorm.” However, Corridore added “the reason for the cancellations are a cause for concern and needs to be explained.”

The problems arose after a switchgear, which helps control and switch power flows like a circuit breaker in a home, malfunctioned for reasons that were not immediately clear, said Georgia Power spokesman John Kraft.

Industry consultants say airlines face an increasing risk from computer disruptions as they automate more of their operations, distribute boarding passes on smartphones and outfit their planes with Wi-Fi.