If you have a laptop with a built-in webcam first check to see if it is turned on. Hold down the FN key and press the webcam on/off button (this will be one of the F1-F12 keys, look for a key that looks like a camera/webcam).
If your having problems with your webcam you should first determine if there is a bug in the distribution or program your using. There are several applications you can test with: qv4l2, guvcview and luvcview are common test tools. Be aware that different webcams might use different video formats, and some of them can be unsupported in some applications.
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Also- you may need to reboot before the webcam will work.