Unless something just "gave out" on your hard drive...
a lot of times, I've found that once they get more than 1/2 full... they start having issues!! In that case, backing up files to a jump drive, CD, or an external hard drive, is the first step.
THEN, go on and delete those files off the internal drive. Once you've "cleaned up" everything... then defragment the drive. This can take awhile depending on how many times you've saved documents, pictures, etc... then deleted them.
I HOPE it's something simple like this. There are a bunch of other things that could go wrong; BUT usually an expert can save what you have on the hard drive, even if you can't get to it from the computer yourself. I'll keep my fingers crossed for ya!!