I can't get my facetime to open and when it does the screen is black?

When I click on my facetime icon no screen comes up but the green camera light turns on. I can also hear when someone is calling but have no way to answer. When I try to do anything I just get a black screen and I haven't been able to open preferences. I also haven't been having any luck with skype either which makes me think it is my camera. I was having this problem before but restarting my computer had solved it up until now. Any suggestions?