- You cannot go back to an earlier iOS legally.
- The chances are your screen has failed.
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.
(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
iPhone Service & Repair - Apple Support