If you can’t see the image from your webcam or you receive a "No device found!" error when you try to use it: press Fn+F10 to turn cycle the camera on, and then restart a webcam application like Cheese. You will now see the image from the webcam in the app.
If your webcam is not working after toggling it on and off with the keyboard key, check the menu of the software application you are using. In the devices menu, determine if it's recognizing your webcam and if so, make sure it is selected as the device to use for that application. If nothing is showing in the devices options, your webcam is turned off or may require hardware repair.
The Fn+F10 combo controls the viability of the webcam on the USB bus at the device level. Applications do not get notification of a new device and will need to be restarted to do device discovery again.