If you go to the plug-in page in the options is it unchecked? If so, check it back on, close Rhino, and re-open.
The other thing you could do is just run the installer again. I don't think you will lose any custom work (like machine set-ups and post-processors), but I would back these up just to be safe. You should find them in C:\Programdata. You may need to make hidden folders visible to see that in explorer.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)