Ensure your external camera is properly connected, and close any programs using the camera (like Skype or Messenger). Press "Ctrl+Shift+R" to refresh the page and update the camera settings.
Also, check that the camera permissions are granted to the website:
Click the lock icon to the left of the address bar.
Set the camera, microphone, and other permissions to "Always allow on this site."
Reload the page to apply the updated settings.