Oblivion for my PC suddenly stops working?

I have had oblivion for a few months and it has worked fine for the whole time but all of a sudden when i try to play it again, it wont work. I am able to get to the main menu, then i can click continue from my last save point, then the loading screen comes up and it says one of the games tips or whatever. Once it is done loading, the program shuts down and a screen comes up saying "Oblivion has stopped working. An error has caused the program to not work correctly" (or something very similar) . Then i am forced to end program (there is only an "OK" button to click). This happens every time i try to play the game. It has never been a problem before. Please help!