This morning when I fired up my computer it wouldn't boot to Windows XP, when I went into the BIOS I found that it hadn't found the master and slave PATA harddrives. I replaced the master one with a clone I keep as a backup but it wasn't recognised aswell.
How is your motherboard coin cell battery? When motherboards get a bit old, these batteries will wear out and one of the symptoms will be the BIOS loosing its settings. If you select autodetect HDDs (or whatever the similar option in the motherboards BIOS is) does it still not detect the HDD?