camera goes blank or half screen. Defective?
iPhone 5 camera goes blank, won't open or shows half picture when trying to focus. Defective. What to do?
iPhone 5, iOS 6, solution please
Basic troubleshooting from the User's Guide is reset, restart, restore (first from backup then as new). Try each of these in order until the issue is resolved.
If the above does not resolve the issue, take the device to Apple for evaluation and possible replacement.