Construction damage to broadband cables and problems with check-in and boarding for German airline Lufthansa led to the cancellation of more than 200 flights at Frankfurt Airport.
Flights for thousands of passengers were canceled or delayed all over the world.
According to the company, on Tuesday in Frankfurt, mistaken cable cutting occurred during engineering work on a railway line. .
Customers flying domestically have been asked to reserve train tickets.
The business announced that customers could request refunds through its website and predicted that things would be back to normal by evening.
Lufthansa tweeted, "We are working on a quick solution.".
Data from the flight tracking website FlightAware shows that over a hundred flights have also been delayed.
On social media, some travelers claimed the German airline stopped using computers to process baggage and instead switched to pen and paper to organize boarding.
One traveler claimed on Twitter that employees at the Frankfurt airport only helped people who had manually checked in.
Others complained that the company had not offered them any assistance and that they were unsure how to get their refunds.
"Lufthansa encourages impacted customers to check the status of their flight on the airline's app or website before traveling to the airport. Up until Sunday, passengers with domestic flights can change to Deutsche Bahn, the airline said in a statement.
An additional statement from a company representative read, "We regret the inconvenience this will cause our passengers.
Frankfurt Airport, Germany's busiest airport, has had flights diverted, but the issue has affected services around the world.
A drill that damaged a bundle of Deutsche Telekom fiber optic cables was to blame for the issue, according to Lufthansa and Germany's national train operator.
This meant that on Wednesday, Lufthansa's check-in and boarding systems jammed, forcing German air traffic control to halt incoming flights.
The DFS air traffic controllers in Germany reported that since Wednesday at noon, there have been about 40 landings per hour at Frankfurt Airport, which is nearly normal traffic.
In a statement, Deutsche Telekom said: "Many customers have already returned to the internet after our technical team repaired two cables overnight. ".
Separately, on Friday, German airport workers are scheduled to walk off the job over a pay issue. .
After a contractor deleted files on a crucial computer server used by pilots last month, domestic flights were grounded throughout the US.
Following the failure of the Notice to Air Missions (Notam) system on January 11, more than 11,000 flights were delayed and at least 1,300 were cancelled.