Ohhh yeah I see. Somewhere along the line your mia_mat got disconnected from it's shading group. Mental ray won't render the surfaces using that shader because well, it has no surface shading information. Middle mouse drag it back unto the material channel in the shading group's attribute editor(under the "Custom shaders" section), it'll automatically make the proper connection. But if you care about the more technical side of things or are forced to do it manually, the attributes being connected are the mia_mat.message to shadingGrp.miMaterialShader. Hope that helps!