As per > How to Troubleshoot iSight
Check iMovie and other app's that use the iSight, it can only be used by one app at a time.
Try another User Account.
Reset the SMC > System Management Controller (SMC)
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.