You may need to install drivers for your webcam. Go to the web-site of your hardware manufacturer and download it.

If it still doesn't work, open your Control Panel and try to enable your webcam from there:
"Control Panel" -> "Hardware and Devices" -> "Imaging devices". Go through the tabs and click "Enable" on your webcam. If it is greyed out and won't let you click on it, try to re-install its driver.