The screen automatically turns off and the phone goes to sleep mode after a certain period of time, which is normal. You can adjust how long you want the screen to stay on.
If the issue is not being caused by an app, check for updates to your phone's software and if an update is available, download and install it.
To prevent the phone from powering off due to high temperatures:
Device Protection may be enabled, which means that after the factory reset, you'll need to sign in using the same
Google Account that was logged into your phone. If you forgot your
Google Account password, use your computer web browser and go to
www.google.com/accounts/recovery to reset your password first.
To perform a factory reset:
The phone will reboot and reset.