In the U.S., American Airlines, Delta Air Lines, United Airlines, Spirit Airlines and Allegiant Air had all their flights grounded for varying lengths of time Friday morning. Airlines said the outage hit many systems, including those used to check in passengers and calculate aircraft weight — necessary information for planes to take off.

United and some other airlines issued waivers to let customers change travel plans. The overnight outage was blamed on a software update that cybersecurity firm CrowdStrike sent to Microsoft computers of its corporate customers, including many airlines.

By midday on the East Coast, more than 2,200 U.S. flights had been canceled and more than 6,000 others delayed, according to tracking platform FlightAware. Worldwide, more than 3,500 flights were canceled. Those numbers were certain to rise throughout the day.

CrowdStrike said it identified the problem and was fixing it, but that didn’t translate into clear skies Friday. Hundreds of thousands of travelers were stranded.

“This is going to have ripple effects for probably a few days at least,” said Jesse Neugarten, the founder and CEO of travel-search site Dollar Flight Club.

That’s because many flights are sold out or have just a few empty seats during the peak summer vacation season, which leaves airlines with less room to accommodate passengers from canceled flights.

Contact your airline

Airlines should rebook passengers automatically, but that could take much longer as carriers recover from the outage, so passengers will have to take more initiative. And be more creative.

“People who are already at the airport usually go up to the help desk, but there’s a line of 500 people. It’s going to take all day,” Neugarten said. He suggested calling the airline and using an international help-desk number, if there is one, to reach an agent more quickly.

Another tactic is to post a few words to the airline on the social platform X. Many airlines have staffers who will help rebook passengers who contact the carrier through social media.

Use your airline’s app — it may have more-current information about flight status than delays and cancellations displayed in the airport terminal.

Check other airlines and airports

In the U.S., Southwest and Alaska Airlines said they were not affected by the outage. However, since it’s summer vacation season, those airlines have a limited number of available seats left.

Neugarten suggested checking nearby airports for available flights, which might require changing airlines.

What about weekend flights?

Long delays and some cancellations could drag into Saturday and Sunday. Check the flight status on your airline’s app or website frequently.

Some airlines are telling passengers to arrive as much as three hours before their scheduled departure times. Many experienced travelers scoff at such warnings; this might be a good time to take them seriously.

The Transportation Security Administration app lets travelers look up the estimated wait time at their airport.

Is air traffic control working?

U.S. Transportation Secretary Pete Buttigieg said the Federal Aviation Administration’s systems, including air traffic control, were not affected by the outage. TSA said its screening of passengers was not affected either.

Can I get a refund?

Passengers whose flights are canceled are entitled to a full refund in the form of payment they used to buy the ticket. That’s true even if the ticket was sold as non-refundable.

A refund may be acceptable to travelers who no longer want to make the trip, but many people just want another way to reach their destination, and buying a last-minute replacement ticket could cost more than the refund will cover.

Am I eligible for other cost reimbursements?

Airports in Amsterdam, Berlin and Zurich had particularly high numbers of canceled flights. When airlines in Europe are responsible for a cancellation or delay, travelers are entitled to compensation under a regulation known as EU261, but that rule has an exception for disruptions caused by “extraordinary circumstances” beyond the airline’s control.

There is no provision for compensation under U.S. law; airlines set their own policies for reimbursing stranded travelers for things like hotels and meals. The Transportation Department maintains a “dashboard” showing what each airline promises to cover.