Exclusive camera access
Also called camera in use · device in use
On Windows a webcam can usually be used by only one application at a time. A second app trying to open it gets an error or a black picture until the first app lets go.
In practice
This is why a camera works in one app and shows a black screen or an error in another. Video tools, virtual camera apps and background effects often keep the camera open in the background. Closing the visible app isn’t always enough if a helper process keeps running.
How to check
Open the webcam test. If it reports that the camera is in use or shows nothing, close every app that might use the camera, including ones in the system tray. In Windows, Settings shows which apps recently used the camera under the camera privacy page.