(1) If your only problem is with camera operation, restart Mac and try again. If the the camera still does not work, the suggestions in Apple's http://support.apple.com/kb/HT2090 will help you.
(Apple has changed the built-in camera's name on newer Macs from "iSight" to "FaceTime" and then to "FaceTime HD." Regardless of the name of your built-in camera, the same info and troubleshooting applies.)
(2) If FaceTime (or any other app) will not launch, use OS X Recovery to repair or reinstall your software.