If your camera is not activating it is usually one of two things:
1) you haven't allowed the camera in the app or
2) the video application is conflicting with another video meeting app.
You can click the little lock symbol to the left and scroll down to site settings and the to the right of camera (or microphone) and select allow. Then reload the page and proceed.
If that doesn't solve the problem, completely restart your computer and open Chrome browser and proceed from there.