As far as I know, unlkess there is a protected boot block in the BIOS EEPROM, the only solution is to remove the IC and re-program it externally. I'm suprised there isn't a boot block you can activate, it would at least allow you to reprogram the BIOS even if it did nothing else. The trouble is there is no standard way to activate it. I would try switching on then repeatedly pressing one of the 'F' keys to see if it pops up a recovery message. You may have to try all the keys but make sure you switch off and then on between each key you try. If that doesn't work, try other keys, such as Esc, DEL, Home, insert and the cursor keys.
Brian.