I have noticed that Mental Ray likes to Un-plug itself a lot, so as a solution to the vast amounts of e-mail I've been getting I decided to create this page so I don't have to type as much =D

Window>Settings/preferences>Plug-In Manager... Now scroll down and look for "mayatomr.mll" if it's not checked then that was your problem, if its not in there, then you're probably on an older version of maya and don't have it and I'm pretty sure you have to pay for it if it doesn't come with your software which would cost you about $999 U.S. Dollars. Once you've got it checked go to your render globals and in the renderer drop-down menu look to see if it's in there to see if maya has loaded it, if not restart maya and then check and it should be there.