Hi
If you have not yet done so, it may help to update your iPhone to the latest software, starting by making a backup:
Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
Otherwise, follow the instructions here, including contacting Apple Support (mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider if necessary:
Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support