It could be a few things, so don't worry just yet. Virus check? Would be the first thing to try I reckon.
Then I would try it on another IDE channel. Turn the PC off (At the wall) and swap the wide (IDE) cable from the broken hard disk with the one from the CD-ROM. Turn the PC on and see what windows finds. You might need to adjust the jumpers on the drive though (But worry about that if it doesn't work).
If you get no luck there, I would try it with Knoppix, see if you get anything there. It's just a CD that you can put in and Linnux boots off the CD. Pretty slow, but handy for checking things like this. You can download it from HERE, but it's 700MB.
Have you had a look in DOS at it? I think it depends what version of Windows you have, but if you press F8 just before the windows splash screen (keep tapping it basically), and there may be an option to get a DOS command prompt. Choose that, and then type D: (or whatever the drive letter is), then type "dir". That will give you a list of all the files on the drive.
There's some things to try anyway...