my built-in camera is not working

I bought the Mac laptop in 2014. I have not used the built-in camera until 3 years ago. Since then it was not working at all. I am trying to get it fixed. I called App Support line. They said there are a few different things that may caused the problem. It maybe software or hardware problem. If it is the hardware problem, it costs a lot to fix. If it is software, then it doesn't cost much. So I am wondering if there is any way to diagnose the problem. Does anyone have any suggestion?