Have you tried holding volume down and power for ten secs, let the phone reboot.. then try it.
Did you drop the device or download any apps before this happened? Change any device settings? Tamper with anything like developer options?? If so, do the inverse of what you did.. see if that clears up your issue... if not see below.
You said your device is stock?? Then try this, delete your third party camera apps, then reboot the device, see if the camera comes online, if so, then reload your other camera apps.
If not...recheck and make sure you deleted ALL third party camera apps. Try clearing data and cache and force closing the camera app in manage apps, all.. reboot again...
If the camera doesn't work now... and you're sure you're not running a rom or something that doesn't support your camera...then i'd suggest backing up your data, and doing a factory data reset.
