Unfortunately, on windows, only one program can use a camera at a time. If you try to use a camera in Meetn that is being used elsewhere, it will show an error, even if you can't see the application that's using the camera. If you know which program is using your camera, go ahead and close it, the refresh your Meetn webpage.

Note: Virtual Camera or recording programs like OBS or ManyCam may be quietly keeping your camera enabled 

If you're not sure which program is using your camera, here are some troubleshooting steps you can try to find which program is using your camera so you can close it:
 

1. Press the Windows key + I to open the Settings menu.
2. Click on Privacy & security.
3. Select Camera from the left sidebar.
4. Under App permissions, click on Let apps access your camera.
5. If an app is using your webcam, you’ll see the message Currently in use under its name.

You can also try fully restarting your computer so that all programs close and when you get back to Meetn, you should see your camera available again.

If still you're unable to find the mysterious application, then you may need to contact Microsoft Support. Please DO NOT google search for their support line though, as there are a lot of scammers pretending to be Microsoft. Please use the built-in Get Help system application that's included on windows and contact the support representative through that application. Press the Windows key or open up the Start Menu and type into the search bar "Get Help" and that will show you the way.