At boot, check the BIOS output to see if it reports the disk as being present, if it does, ignore this bit. If not, you will have to open the case and try reseating the cables to the drive at both ends. If it still doesn't work the disk, or the controller is probably duff. Quick check is to swap the disk over to the second IDE connector on the motherboard and see if BIOS reports it there.
If BIOS reports the disk OK then there are 2 possibilities. You may have a hardware fault in which case it's replacement time, or you may have scrambled the contents of the disk. Boot using your windows startup floppy (if you don't have one, you can download one from click here ). Run scandisk from the floppy and see what it reports back with. There is also a utility at click here which I haven;t used yet which may be of some use or do your own search for file recovery software.
If the files can't be recovered, it's fdisk and reformat time unless someone can suggest an alternative.