Is there a chance you have you disabled your camera which is stopping your live video from displaying?
Perhaps you'll see a message like this:
Solution: Allow your camera
If you are using Firefox, click on the site identity icon (the lock symbol) in the address bar and click Allow
Click the refresh page icon on the top right of the address bar:
Please be patient while the page reloads...
Then when you're ready to re-enter your class with the new settings, please make sure you click Allow when prompted to start your camera.
Now you should be all set. Enjoy your class!