Built In Mac Camera Isnt Working On Oovoo My Web Cam On My Mac Isnt Working And Idk How To Turn It Back On...HELP PLEASE?

My web cam on my mac isnt working and idk how to turn it back on...HELP PLEASE? - built in mac camera isnt working on oovoo

I have a Mac and I have only 2 or 3 months. I think I accidentally turned off the camera built into it, because when I was in Photo Booth, it was an image from a camera with a slash through it. Who knows how to reactivate? PLEASE HELP ME:)