Perhaps you have another application running that's reserving the camera. Such applications may include Skype, instant messaging clients such as Google Talk, and corporate communicators such as Microsoft Lync.
Close all open applications that may be reserving the camera (using Task Manager to end the applications is an effective way). If that doesn't help, try restarting the computer.
Some laptop manufacturer's software may disable the webcam by default. Make sure that the webcam is enabled by default in those softwares.
For example Lenovo has Communications settings as follows:
If you use Google Chrome as your browser, you need to allow RecRight to use the webcam from the dialog below the addressbar: