Click in the menu bar.
About This Mac > More Info.. > System Report > Hardware > USB > FaceTime HD Camera(built-in)
Make sure that camera is listed.
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".
If this does not help, contact Apple.
(Over time, 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 Mac's built-in camera, the same info and troubleshooting applies.)