A webcam that isn't working is usually caused by a permission block, another app using the camera, or a driver issue. First, check if your browser has camera permission — click the padlock in the address bar and set Camera to Allow. Then close any other app that might be using the camera (Zoom, Teams, FaceTime, OBS) since most webcams can only be used by one app at a time. If the camera still shows a black screen, check your OS privacy settings: on Windows go to Settings > Privacy & Security > Camera and enable access; on Mac go to System Settings > Privacy & Security > Camera. For external USB webcams, unplug and replug the cable, try a different USB port, and make sure drivers are up to date in Device Manager.