Pre-repair procedure.
First thing I advise is to try a Forced restart
Force restart iPhone – Apple Support (UK)
If no better you could Backup, restore to factory, then restore from the backup:-
If necessary set up without restoring backup and test at that point, erase again, then restore from the backup:-
(If the phone needs attention you will have to do this anyway)
See here for the resources you need. Allow a couple of hours:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Otherwise arrange a repair for hardware failure
Click here > iPhone Repair & Service - Apple Support
Or
Click here > Contact - Official Apple Support
Anything other than Screen or Battery may be a replacement phone and the price is in the article under "Other damage"
See also: Get your iPhone, iPad, or iPod touch ready for service - Apple Support