I had a similar problem once. My laptop lost power while it was starting up, and it lost the file system. As Shinrai said, chkdsk may be able to fix it, but if not, there are many programs you can use to recover a hard drive. I can't remember which one I used, but they all do about the same thing. They look through the data on the hard drive for patterns, and use that to figure out where files are. All of your data is most likely still there, Windows just can't find it.
To run chkdsk on your system, you have a couple options. If you have a Windows disc, you can boot into that. Otherwise, download RC.ISO from this page and burn it to a CD. Note that this is not the same as burning a data file, you need to make sure you're burning the ISO as an image.
Once you have one of those, put it in the computer and boot from the disc. To do this, keep an eye on the bottom of the screen as it boots. Early in the boot, it will display
[key] for boot options (On my HP computer,
[key] is F9). Choose
Boot Device, and select your CD drive. Once it boots, you will be able to choose
Recover Console. You may need to log in at this point.
At the Recovery Console prompt, type in
chkdsk /r. Let the chkdsk run uninterrupted until it completes. If it gives any errors, let us know. Once it finishes, type
exit, and then press
Enter to leave the Recovery Console and restart your computer.