Shiona is a technology reporter.
The Holiday Inn is owned by the Intercontinental Hotels Group.
IHG, which has some of the world's largest hotel chains, said it was looking into unauthorized access to a number of its technology systems.
The company that manages the Holiday Inn and other hotels in the UK said its booking channels and other applications had been disrupted.
The company said it was working to restore all systems.
IHG said it was assessing the nature, extent and impact of the incident and had implemented its response plans.
The company is in the process of telling regulators.
The company said in a statement that they would be supporting hotel owners and operators. The hotels are still open and can take reservations.
Many people trying to find a place to stay are not happy.
The BBC is not responsible for the content of external sites.View original tweet on TwitterWhat is going on with your system? For at least 19hrs. Phones and apps not working- afraid to book anything. No customer service at all
— Kim Sweat (@rsweatark) September 6, 2022
There was no mention of a loss of customer data.
Most of the speculation points to the fact that it was a Ransomware attack.
The Holiday Inn in Istanbul was the target of a data theft.
There is no known connection between the two attacks.
More than 1,200 of the hotel chain's franchised hotels in the US were affected by a three-month security breach in the summer of last year.
Increased scrutiny on appropriate defences against cyber attacks, particularly on western financial institutions, has arisen in the wake of Russia's invasion of Ukraine.