First check that all third-party (non-Apple) apps you have added are the latest versions compatible with your system. Then use your Mac's Disk Utility to repair permissions, restart, and and check camera function again.
If the camera problems remain, see http://support.apple.com/kb/HT2090 to help you find and correct the problem.
If the trouble persists, use Disk Utility to repair permissions. Then download and apply the latest Combo Update for your OS, currently http://support.apple.com/kb/DL1581. Repair permissions again immediately following the system restart that completes the Combo update.
Properly applying the Combo version of updates can sometimes correct software problems, but, if your trouble remains, you may need to reinstall OS X in its entirety. Then test your camera one more time before reinstalling any third-party items.
You're most welcome.
If reinstalling doesn't help, when you post back here also tell us whether your cam works to record with Apple apps like Photo Booth and QuickTime Player. If not, verify the symptoms are EXACTLY the same in more than one user account. (Do NOT use the "Guest" account for this test.)
We await your status update.