First try a Forced restart
Force restart iPhone – Apple Support (UK)
If no better
Backup, restore to factory, and test, then restore from backup:-
(If the phone needs attention you will have to do this anyway)
If you can import your media to Photos or iTunes in a computer and copy important documents to a cloud server it will give you some resilience in case the restore from backup fails, or you need to ditch the backup and start as new.
See here:
(1) How to back up your iPhone, iPad, and iPod touch - Apple Support
(2) Restore your iPhone, iPad, or iPod to factory settings - Apple Support
(3) Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If the problem remains then do step (2) again but set up as new without restoring any backup.
If it fixes it then it is probably a rogue app.
Otherwise arrange a repair
Click here > iPhone Service & Repair - 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