When I open a project, or start a new one, when I go to use the rotobrush, there is no green circle showing where my click will affect; it's just the mouse pointer. This is so frustrating, and I've restarted the program, restarted my computer, but it's still just refusing to show the brush! How on earth do I get my green circle back?
Nobody can tell you anything without proper system info or other technical details like version of AE or your brush settings.
It sounds like you are not using the layer panel or you have turned off viewing the controls at the bottom right corner of the panel or you have turned off view layer controls in the panel options.