You can get 3rd party camera apps through the Apple App Store, many available there. However I would try a forced restart first and if it does not help I would restore the phone. If the phone is defective, a replacement would be your only option.
System Restore. First backup your device using iTunes. Then import your photos to your computer and copy all important data. Try restoring from backup first and testing it. If it does not help, you may have to restore as New and reconfigure from scratch as the backup could be corrupted. For this reason it is important to have your photos and data saved separately from the backup.
https://support.apple.com/en-us/HT201252