Sounds like a hard drive failure to me. Did you hear any strange noises from your computer recently, anything out of the ordinary? Not to accuse you of anything, but have you dropped it recently? Also, last thing, how much free space do you think you had on your drive last time you checked? Any system performance slowdown?
Otherwise, it could just be that your OS got corrupted.
The only reason the HD failure is my first guess, is that it just happened to me and my friend within the past week.