Not having video in your class can be quite frustrating. Fortunately, it's usually fairly quick and simple to fix.
The most common cause is that you've accidentally missed the notification asking if you'd Allow the web browser to access your webcam.
Other possible causes are:
Another application or window is using the camera. Some computers don't support multiple apps accessing the camera simultaneously. Closing other tabs or apps will help.
Your browser is a little confused...Try quitting the browser, reopening and re-entering the class. You'd be surprised how often closing all other apps/windows can help!
A firewall is preventing live video...Click here for resolution
The camera itself may not be working. You can check whether your webcam is working or not here on webcamtests.com. (Please bear #1 in mind - you may need to test this after your class has finished).
Remember, in most cases you can teach the class quite well with audio only. In cases where having video may not be a top priority, it's best to not to waste too much class time trying to solve the issue. If it looks like it won't be so straightforward, it might be best to stop and try again after the class.
This also applies to students. If there isn't a simple solution, you should send them this article, or copy and paste the content, so they can try the steps out after class.