I have this issue sometimes too. With the exception of wiping my phone I've tried all the simple solutions, that is reset settings, reboot, power off for a while, open from lock screen, leave camera app running in the background etc...
It seems like none of these worked consistently. On occasion I'd be able to take a few photos, but as soon as I clicked on the thumbnail to view the image and returning to the camera, the rear camera didn't work again. The video works consistently, and the front camera works consistently too. This issue tends to coincide with my flashlight not working, when I hit the flashlight button it doesn't even change state.
This is what worked for me (but i'd only do it as a last ditch effort):
My phone is slightly bent (just barely), enough that the screen sort of clicks if I push on the corner. At some point I tried to bend it back by pressing on the edges of the screen a little above half way up the phone while pulling the top towards me with my index fingers and pressing the bottom of the phone against my stomach. I do this with a _reasonable_ amount of pressure, it's forceful but not enough to do any damage. It turns out I hear a little click at which point I stop. Afterwards my flashlight and camera work consistently for a while, at least several weeks.
I must jar my phone occasionally, or put pressure on it in my front pocket which makes things go bad again. But I've done this maybe a dozen times and never made things any worse.
Good luck!
