when something like this happens to me what i do is: in new scene unload then reload mental ray. Start a new scene, set mental ray as renderer, then do a quick render test on a cube or something. If it works great, now just import your scene into the new scene and mental ray should work. This has fixed a whole number of different mental ray issues for me, Although your milage may vary.
I get this all the time. It varies from model to model but it always happens to me when my AO samples were set too high. My latest character didn't like the samples set at 128 so toning it down to 96 worked. Lame IMO because I'd like to set everything to 256 if I could.
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@Junkie_XL: i'm glad that i'm not the only one having this problem.