More than 200 flights in and out of Belgium were cancelled or diverted on Wednesday after a power surge disabled the operations of Belgocontrol, the domestic air traffic controller.
At Brussels Zaventem, the country's main airport, queues formed at the desk of Brussels Airlines, the national airline, as passengers sought to re-book flights.
The suspension prevented flights from arriving at or leaving all Belgium's airports, although planes could still cross at higher altitudes, where the upper air space is controlled by the European traffic authority, Eurocontrol.
Air traffic operations gradually resumed from 14:00 local time, but delays could still be expected and some flights would probably be affected on Thursday, Brussels airport said on its web site. It advised passengers to check flight information online or to contact their airline for more information.
A Belgocontrol spokesman said the power surge "also took out our emergency generators, which is why we had to clear the airspace."
French and German aviation authorities said there had been little impact on flights in and out of their countries.
Belgocontrol is responsible for controlling the civil airspace from ground to 24,500 feet (about 7,500 m) above Belgium and for the space between 14,500 and 24,500 feet above Luxembourg.
The airports of Antwerp, Liege and Ryanair hub Charleroi reported delays, cancellations and diversions. Luxembourg Airport said the outage had had no effect on its operations.