Hello guys, had same problem after i updated to 8.1.2, the back camera and the flash stopped working after 2 days. Just fixed it 2 mins ago.
1 step make sure it is not a hardware problem - use the camera with face time that is the only app that worked for me (try to call someone using face time and switch back and forth the cameras to hopefully see that it works.
2 step hold home and lock button till the phone goes off leave it for about a min off then turn it back on.
After this try out the camera app.
If it doesn`t work the first time just repeat the steps with face time etc. but before this do a backup restore that you think worked well with your phone.
This worked for me.
I kinda think it is maybe a software - hardware conflict between multiple app and the actual camera because after the update my camera worked fine but then i used an app called retrica that has lots of cool filters for photos, but i think it stresses the phone`s camera and maybe causes some conflicts.
Hope this will help you..