Ryanair has confirmed it was hit by a "system outage" on Saturday morning.
The issue caused delays at airport check-in and boarding across its flights.
A number of passengers on Twitter said they were unable to check-in online or at various airports.
One said: "Daughter unable to check online last night as website down ..queued for an hour at Gatwick now to pay £55 penalty while staff are struggling with system down."
Another user said: "My return booking from Nice to Stansted today has disappeared from both app and website."
While another wrote: "Why is your app not working? I have three flights booked but apparently I have none. Why?! I am supposed to be flying home today, this is a joke.
"I need a human not a bot!"
The carrier said the system has since been restored and flights are operating as normal.
It has apologised for any inconvenience caused to customers.