had a similar problem recently (ish) with old comp, BIOS was screwed by accidental bad flashing. dont even try to correct this until ALL else has failed, it can screw it up, and if its not the problem you have another one.
if mobo is new/under warranty you can replace it or get a new BIOS chip sent to you (manuf' might even do this anyway). else, you may still be able to flash it, check manual etc for other ways, but this is how i flashed mine (ONLY APPLIES TO AMI BIOS's, maybe another way for other ones)
DL the flashfile.
rename the flash rom to AMIBOOT.ROM
insert into floppy, and turn on.
configure BIOS as normal.
I really really mean it, absolute last resort only. that means do not do it until the mobo is heading for the bin.