Flights in and out of Switzerland were suspended for hours and Swiss airspace closed because of a computer failure at air traffic control service Skyguide.
Geneva and Zurich halted early flights but Geneva airport announced air traffic was gradually resuming.
Skyguide said a technical malfunction in the early hours of Wednesday meant Swiss airspace had had to be closed to traffic for "safety reasons".
Some flights were being diverted to Milan in northern Italy.
Skyguide spokesman Vladi Barrosa told Tagesanzeiger news website that the problem appeared to be a hardware fault and they did not believe their system had been hacked. The company later announced that the "technical malfunction at Skyguide has been resolved" and airspace closure lifted at 08:30 (06:30 GMT).
A Flightradar image of air traffic across Western Europe showed a substantial hole in flights over Switzerland early on Wednesday.
According to Eurocontrol there is a system failure in Switzerland right now:— Flightradar24 (@flightradar24) June 15, 2022
LSAZ and LSAG (Zurich and Geneva). Zero rate regulations due to system failure. High delays.
Live map https://t.co/OcDzZ0ssJD pic.twitter.com/93HsF4J4HL
Flights had continued as normal at Basel-Mulhouse airport because although Basel is in Switzerland the airport itself is on French territory and covered by French air traffic control.
Geneva airport said several flights had been cancelled but that air traffic had gradually resumed from 08:30.
One passenger at Zurich posted a picture of all the flights unable to take off.