In Switzerland, no planes could take off and land for several hours on Wednesday morning.
The reason for this was the failure of the Skyguide computer system, which monitors Swiss airspace and the adjacent airspace.
The company announced that Swiss airspace was closed.
This is no longer the case.
Skyguide was able to fix the problem, and the company announced on Twitter that it regretted the failure.
It was not a cyber attack, said Skyguide spokesman Vladi Barrosa, the German Press Agency.
The cause was a hardware problem in the IT network.
All airports in the country were affected by the failure, including Geneva, Zurich, Bern and Basel.
In Zurich, too, at the country's largest airport with 300 daily departures, the planes had to remain on the ground for the time being.
There, the operator announced that flight operations were initially running at a capacity utilization of 75 percent.
A picture was circulated on Twitter early Wednesday morning showing a flight board showing that all flights were affected.
Passengers complained on Twitter that they had already been guided through check-in and were now stuck at the airport.
Incoming long-haul flights were diverted to airports in neighboring countries, such as Lyon, Milan and Vienna.
Take-offs and landings at Swiss airports have now been observed again using the Flightradar24 tracking tool.
Numerous planes were waiting at the airports beforehand to continue their journey.
So far it is still unclear what effects the computer failure and the delays will have on European air traffic.
Aviation is currently under pressure anyway because there is a shortage of staff at the airports.
Lufthansa alone had announced that it wanted to cancel hundreds of flights in the summer in order to relieve airports.