long time since i posted, but i'd check the POWER CABLE and IDE/SATA cable thats plugged into the hard drive first... remove and reconnect these (and on the motherboard too)... check in bios for the drive again... if its not there then...
First of all, I am suprised there has been no backup system for events such as this.
There isn't a lot you can do.
Did you change the jumper settings when you tried it in the other system?
What brand is it?
Is it SATA or ATA?
In some cases when Windows can't examine the contents of a drive, a bootable third party program may
be able to help you recover your data. But, as said above, a backup would be better.
If the electronic interface has been hosed, you may need to use a professional recovery service. Seagate can help there, and they provide a free consultation. It's very expensive.
This topic has been discussed a number of times on HWA and you may find some solutions by searching in the HWA search box
If it's just file corruption, a simple data repair/recovery environment called Knoppix may be able to read your data so you can copy it to another disk, or you may need a more sophisticated recovery program.
If you have another computer then try to remove the hard drive and install in the second computer as a slave drive. Then you should be able to access the files on the hard drive. It is really a very simple process if you have another computer. I have had to do this a couple of times due to hard drive failures (more likely just a windows failure that won't let you get anything off the hard drive). When I connect to another computer I can read all of the data on the hard drive.
Hope this helps.