More than 200 flights were cancelled at Frankfurt airport after construction work damaged broadband cables and caused check-in and boarding problems for German airline Lufthansa.
Thousands of passengers had their flights cancelled or delayed across the world.
The company said engineering works on a railway line mistakenly cut a bundle of cables in Frankfurt on Tuesday.
It has asked customers flying on domestic flights to book train tickets.
The firm said customers could ask for a refund on its website and expects the issues to stabilise by the evening.
"We are working on a solution swiftly," Lufthansa tweeted.
More than a hundred flights have also been delayed, according to data from flight tracking website FlightAware.
Some travellers on social media claimed the German airline switched to using pen and paper to organise boarding and was not able to digitally process baggage.
One customer tweeted that Frankfurt airport staff were only assisting individuals that were manually checked in.
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