Thank you @Gofast for your quick reply!
As there is no setting available in the phone enabling you to adjust the functionality of the power key and a software repair involves you having to power off the phone (which is not possible since the power key do not work), I suggest that you send your phone to an authorized service center to be examined and repaired.
For further instructions on how to send your phone to a service center, please get in contact with your local support team.
