Back Camera not working and the flashlight button is disabled

Hey there NekoCat,

It sounds like you cannot use the rear camera or the flashlight on your phone. I would start by closing all the running apps on the device:

iOS: Force an app to close

http://support.apple.com/kb/ht5137

Double-click the Home button.

Swipe left or right until you have located the app you wish to close.

Swipe the app up to close it.

When you have done that and restart the phone and test the issue again:

iOS: Turning off and on (restarting) and resetting

http://support.apple.com/kb/ht1430

Thank you for using Apple Support Communities.

All the best,

Sterling

Camera has probably failed, not much can be done to resolve that...

See iFixit if the phone is out of warranty.