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.
You can find more at:
http://www.ideasonboard.org/uvc/
Also- you may need to reboot before the webcam will work.