Q: the camera on my iMac won't turn on for skype

Have you checked in Skype>Preferences>Audio/Video - is your camera listed there for selection?

Also, what version of OS X are you running? If you've recently upgraded you OS it's possible you have an incompatible version of Skype installed. Skype gets updated regularly - check Skype>Check for Updates to see if there's an update. The current version of Skype is v6.17 (477).

If your iMac camera doesn't works with Apple camera apps either, see http://support.apple.com/kb/HT2090

(If you are using OS X 10.9.x, http://discussions.apple.com/docs/DOC-6431)