First: close every app that might hold the camera
Close every app that might have the camera open: OBS, Zoom, Teams, Discord, the Camera app and other browser tabs with a camera page. Then start the camera again. On Windows, error 0xA00F4243 (or 0xA00F4289) means exactly this: another app or process is using the camera. Our webcam test says the browser could not open that camera.
On Windows: find the app and free the camera
Right-click the taskbar and open Task Manager. Find the apps that use the camera, right-click each and choose End Task. Background apps count too, such as a meeting app that starts with Windows.
If the camera still won’t open in one app, check the permissions in Start, Settings, Privacy & security, Camera. Camera access and Let apps access your camera must be on. Let desktop apps access your camera must be on for browsers, OBS and most meeting apps. If Camera access is off and greyed out, an administrator has to turn it on.
Security software can lock the camera too: Microsoft lists error 0xA00F4246 for a camera locked by security software. Check your antivirus program’s camera setting.
On a Mac: follow the green light
A green light beside the built-in camera glows while an app is using it, and goes out once you close or quit every app that can use the camera. Quit apps one at a time until it goes out, then open the one you need.
If the camera still won’t appear in that app, open Apple menu, System Settings, Privacy & Security, Camera and turn the app on. Apple notes that you may have to quit and reopen the app first. If you use Screen Time, check that the camera is allowed and the app has time left.
Using the camera in OBS and a call at once
If you want your camera in OBS and in a meeting at the same time, let OBS hold the real camera and choose Start Virtual Camera in OBS. Then pick OBS’s virtual camera as the camera in the meeting app, so the call gets the same picture as your stream.
Test it again
Run the webcam test after closing the other app. If it now shows a picture but OBS or a call still doesn’t, the problem is in that app’s device choice: see webcam not detected or black.
Questions
The camera light is on but no app is open. Something in the background still has it. On Windows check Task Manager; on a Mac quit the apps that can use the camera, or restart.
The webcam works in one app but not another. On Windows that matches the in-use and permission errors above: free the camera, then check Let desktop apps access your camera.