The first troubleshooting step you should try is to restart your computer. Sometimes a single application can take control of your webcam device and not let any other apps use it, but you can release that control by restarting.
Here are a few other steps you can try if the problem persists after restarting
# Microsoft has also created a detailed article. It is for Windows 10 and you may want to click here to explore potential fixes. Pay special attention to the Privacy settings. This is a new feature in Windows 10 that may block access to the camera or microphone, often without any error message. To check the privacy settings in Windows 10 click on this link. Besides the Microsoft Store Apps also turn "on" the "Allow desktop apps to access your camera" if you will be using desktop apps such as Zoom. You may also need to check the microphone privacy settings.
# If you own a commercial/business class device such as HP Elitebook/Probook/Zbook, make sure that you check your BIOS/UEFI settings - ensure that the Internal Camera/WebCam is checked and enabled there. Access the BIOS/UEFI through F10 key during power-up --> Navigate to "Advanced" tab --> Built-In Device Options
# If you only experience a WebCam issue in a particular app (such as in Zoom, MS Teams, Skype, etc...), check the application's settings or the vendor's article for any specific in-app settings.