I don't know what caused this, it's the second time that it's happened. The first time, I just merged the entire scene to a new file to fix it, but I really don't want to do this every time it happens.
I noticed it happened this time after I imported an OBJ into the scene which I had sculpted/baked out in mudbox, now whenever I try to bake an AO map using Mental Ray everything just turns out completely black. Even if I do a render of the scene, it's black. It worked fine before. The only light i have is a skylight.
Any help would be appreciated.