1. Go to Settings : Apps : All : Camera.
2. Force stop the app.
3. Clear all data
4. Restart your Nexus 7
….If this did not work try again. If you still have a problem try to boot to safe mode by holding tap and holding “power” on the power menu. You may also try a factory reset. If all else fails I think you should claim your Nexus 7 in warranty as it is probably a hardware fault!