My iPhone 4s camera only works on FaceTime?

When I go onto snapchat it says my camera restrictions are on but they aren t. On Twitter instagram and normal camera it is just a black screen but on FaceTime both cameras work

You should check Settings/general/restrictions and see if they're any on for the camera or go to settings/privacy and see if the apps you mentioned have access to the camera, if they don't, that's the problem.