I am using Chrome:
The first thing to do is to click on the little picture of a camera shown in the picture below. If you are not sharing a camera at all, it may be shaped as a microphone.
When you click on the camera you should be able to see the menu in the picture below. You want to make sure that you are allowing, rather than blocking, your camera and microphone.
Also, if you have more than one microphone or camera available to you, you need to make sure that the correct camera and microphone has been selected.
If you click on the padlock highlighted below, you can see everything that is allowed and not allowed on the page, just to check that you are not blocking either your camera or microphone.
I am using Firefox:
When you open the lesson space in Firefox, it should prompt you to enable your camera and microphone. As shown in the picture below. You need to select 'share selected devices' when prompted. It is important to make sure that you are selecting the correct microphone and camera, if you have more than one to choose from.
When everything is working properly, you should be able to see a little picture of a camera and a microphone at the top of your screen, as shown below. If you click on this, it will show you exactly which camera and microphone you are allowing.