Hi!
I would recommend:
1.Try restarting the iPhone by "Sliding to switch off" and powering back on again
Restart your iPhone, iPad, or iPod touch - Apple Support
2.Please ensure a recent iCloud or iTunes backup is made/available before proceeding.
How to back up your iPhone, iPad, and iPod touch - Apple Support
3.Try updating to the latest iOs if not running latest version
Update your iPhone, iPad, or iPod touch - Apple Support
4.Close the Camera Application and re-launch again.
How to force an app to close on your iPhone, iPad, or iPod touch ...
5.Reset All settings (Settings-General-About-Reset)
(Do not erase all contents and setting in the above step, all reset "all settings"
6.Restoring the iOs to factory settings and setup as new.
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
The next step is quite critical
7."Before" restoring any backup, try take a photo, if the issue persists on a factory reset device without the backup restored, you most probably have a hardware failure.
If the issue is resolved and the camera works: Try restoring from iCloud.
Call AppleCare for further assistance:
Contact - Official Apple Support
Hope this helps
Basil