It all depends on how far gone it is. Can you hear it spin? If you place a finger on it, is it vibrating? The best place to start is by slaving it into another PC and seeing if Microsoft's Checkdisk can find and fix any errors. The Command Line syntax will be chkdsk X: /r where X is the drive letter the other computer allocates to it.

Even if that fails to restore the disk back to full system capability, it may still be possible to rescue your files. If the disk doesn't even spin, sometimes a circuit board from an identical disk can help to get it going again.