You need to close all applications (video editing tools, instant messengers, etc.) that may be using the webcam. If you can't find all opened programs, press the Alt + Ctrl + Delete key combination, open the Task Manager and you will be able to close them using the End Task button. If the problem is not solved, you can either reinstall the drivers or run the SplitCam utility which allows multiple programs to use the webcam at the same time.
