Firstly, if you look in the RLS window, the bottom left has a button with a camera symbol. If you click it, it will turn your video feed on/off. Sometimes it's as easy as clicking this button.

But the underlying reason this happens is due to another program (like Zoom, Skype, etc) appropriating the use of the camera on that computer. To resolve it, quit all open programs that utilize your camera (including the RMS sofware), and then proceed to relaunch only our software. You can also check to make sure the correct camera is selected in the RMS software prefences, under the "Live Share" tab.