Currently Being ModeratedMay 27, 2013 7:50 AM (in response to littlemissblue)
(1) Then check the next possibilities: your problem may be either iMovie or user account-specific.
(2) Troubleshoot with every applicable suggestion in http://support.apple.com/kb/HT2090
(3) If iMovie is the only webcam-enabled app (try QuickTimeX, too) that gives trouble in any user account you test, the problem is application specific to iMovie.
Try resetting iMovie preferences. If you need help, the method is as follows:
• Navigate to your Home (~user) / Library / Preferences folder.
• Throw away any files that contain com.apple.iMovie.plist.
(There may be more than one .plist filename containing "iMovie".)
(4) If resetting preferences does not correct your iMovie problem, use Disk Utility to repair permissions and then reinstall iMovie from your iLife DVD. Repair permissions again after reinstalling iMovie, and then launch and test iMovie again.