First, try loading the BIOS’ default settings and saving them, then powering off and back on. You should be able to do this in the BIOS configuration and/or with a key during POST (watch the screen when booting; it should say to press some key to enter Setup, and perhaps another key to load defaults).
Another way to load the defaults is to reset the CMOS via a jumper on the motherboard (check your motherboard’s manual; it’s usually by either moving a jumper cap from one set of pins to another, or shorting out two pin-heads with something metal like tweezers).
If resetting to the default settings doesn’t work, try re-flashing the BIOS (did you see any errors or did the power go out or was the computer reset during the last flash?) You should get another copy of the BIOS image and flashing utility, just in case (your motherboard may also have a built-in flashing utility that can be used by pressing a key on boot). Try doing it from a floppy disk or USB drive. (Don’t forget to load the defaults after flashing and before making changes.)
If it still doesn’t work, then try flashing an older version of the BIOS, eg the backup copy of the version that worked which was made when you flashed recently. (Again, don’t forget to load defaults.)