This can sometimes happen if the system is experiencing a hardware problem that causes connection problems. View your MacBook's hardware information by openning "System Information" from the Utilities folder found inside the Applications folder. Expand the Hardware section, and then click "USB." Click the "Built in iSight" section, located under "USB High-Speed Bus," to find out whether the camera is detected or not.
If the camera is not being detected, sometimes just restarting your computer can solve a temporary connection problem however, if Photo Booth is still not recognizing the camera, you need to reset the System Management Controller.
Have a look here http://support.apple.com/kb/HT3964?viewlocale=en_US&locale=en_US
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