You will almost certainly be recommended to Boot into Recovery & Wipe Cache, so you may as well.
As for the power button, you can test it using Samsung Members app, click on Support & run the test for the Power Button.
If your device is constantly restarting, or shutting down, in addition to Clearing Cache, you can try Booting into Safe Mode & see if continues with the same behavior.
Last resort before visiting a Samsung Service, would be to Backup your device with Smart Switch & Factory Reset. See if that fixes the issues.
Hi tabithafroude, please follow the steps provided by RedDogRabbit. Please keep us posted if your shutting off issue is resolved. Thank you RedDogRabbit.
