InterContinental Hotels Group PLC (IHG) confirmed its booking website, app and systems were reactivated on 7 September after a cybersecurity breach that was first disclosed on 6 September.
The London-listed hotel chain recovered service at its Reservation and Customer Care call centres and all its systems have been restored.
A statement said during the disruption IHG-branded hotels continued to operate and were able to take reservations directly.
IHG insisted it is carrying out further steps, as part of its recovery and assurance plans, to improve security measures and prevent similar incidents in the future.
External specialists are investigating the incident and are yet to find any evidence of unauthorised access to systems storing guest data. The access has been passed on to law enforcement for further investigation.