Have you checked that the IDE ribbon cable in the back of the hard drive is firmly pushed in. The same with the one attached to the motherboard. Also check that the power lead is firmly pushed in.
When you boot the machine, go to the bios and check that all primary and secondary devices are on auto detect. This just may find the hard drive again. Have you heard any mysterious clicking noises from the computer?
The usual method to try and save data from a suspect hard drive is to fit the suspect hard drive as a slave into a different machine. Then try and copy all the folders over to a know good hard drive.