Hmm, while its not likely, but if for instance the mainboard battery has failed it could not be remember bios settings and could have reset the boot order.

Either way I would look at the bios first to A) Make sure all internal drives are seen and B) to recheck the boot order.

If you cannot see one of your drives, this could be a drive failure or something simple like a loose power cable or signal cable, If its SATA drives sometimes the power/signal attachements aren't that secure and can come off. If you can see all drives , check the boot order, if all drives are seen and the boot order is correct, you might have messed up the master boot record(MBR), if this is the case follow these instructions :-

http://www.lancelhoff.com/how-to-fix-vista-mbr-repair-broken-vista/

Jon

DA Portfolio - http://jonj1611.daportfolio.com/