There are presets that you can check to reset mental ray to default settings. I am assuming that you changed something in the global mental ray settings. If you changed something at the component level then I am not sure what the best way to set those back to default short of exporting the model and re-importing it.
In maya 2010 the presets are under the quality tab of the global render settings panel.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675