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You may find the steps in this article helpful for troubleshooting the camera on your MacBook:
Since you've determined that the behavior isn't user specific, try these steps:
3. Find out if the computer recognizes the iSight
Check System Profiler (in the Utilities folder, inside the Applications folder). Under the USB header, check to see if the iSight camera is detected.
4. Reset SMC or PMU
If your built-in iSight camera is still not behaving correctly after trying all these steps, you may need to contact Apple or an Apple-Authorized Service Provider for service.