Is there away to fix a hard disk which is not known by bios? My dell latitude C640 laptop was sufferin from overheating (fan not workin n so it was restarting.) I took the hard disk out to save my important data but it was not recognized at all. Anyway to fix it?

Did you have a bios password enabled on the c640? if yes, the password transfers over to the hard-drive as well and will need to be removed first.

If no, then your drive is probably failing, mechanical or firmware.

more likely a damaged board on the drive that has failed
first option is match up the board

Not ATA password as disk is not recognised at all. If your laptop was overheating then possibly the PCB is damaged. Or possibly worse, but as this is NOT a logical failure there is no software available for a quick fix. Consult a professional, and at the very least get an accurate diagnostic.