This is one of the nastiest problems...
I'm sorry, but if the BIOS fails to detect the disc, there is nothing you can do with software: the hdd hardware is damaged.
However there may be a chance: You need to get another hdd, exactly the same model, and replace the controller (the board that has attached below the disc). It's a bit tricky, but can be done. After this, if the hdd is properly detected, then software may be able to recover files, or even the file system.
If this still fails, then the disc itself may be damaged (or the heads, or drive, or...) then your only option is a Data Recovery Center (expensive, but if your files are so valuable...)
If someone else knows how to fix this with software I'd love to hear... If at least the BIOS detected the disc, there are programs that are able to recover files from very damaged file systems...