Camera

Ensure you ate in the latest iOS .You may have a hardware problem. 

First try a forced restart

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

If that does not help the  next procedure would be to backup then restore your device to factory, and setup the device without restoring a backup, to see if the issue is replicated. 

Then restore the backup. If you need to send the phone in for repair then backing up and restoring to factory would be needed anyway.

Here is a guide for those steps

Backup and restore

(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

If that fails arrange a repair

https://support.apple.com/en-gb/iphone/repair/service