As GtWalling posted, to better assist in thing need a bit more info......What rom your using would be a nice start. Rom, Phne model, etc, some of these things can be vital so others can more help in said situations or problem.
For those of us with s3's we are plagued with having camera issues, namely the dreaded "Cannot connect to camera" issue, but this is mainly in Adroid L of any sorts. All roms with the s3 and android L have this same issue, be it cm12, or any other rom using android L, camera just does not work correctly.
As I hear it they are close to having a fix, but for now if you want a working camera and are on L, I would suggest reverting back to KK rom of choice, until they fix it. I am currently on KK myself, until they fix the camera. Once they do android L will be my choice of daily usage.