Get screen or adapter that fits to integrated GPU output (VGA?) and use that to fix BIOS settings, also resetting settings may help.
Few other things if you are desperate or BIOS is broken:
You could try resetting BIOS or blind reinstalling/upgrading it, BIOS hotswapping is also thing I used to bring broken things back alive. If these methods fail you can try writing bits by hand but that will not be easy thing to do, a lot of work if you don't happen to know some developers worked on your model.
Disconnect power supplies first.
First try to find small button or jumer wire labeled reset, clear, cmos or something like that. When found press and keep it down for few seconds. BIOS settings should now be in factory defaults.
If you can't find reset button then you can locate and disconnect RTC battery. Keep battery disconnected at least for 30 seconds, you could also try to start computer without RTC battery.
Some chips have nice feature to upgrade BIOS automatically from CD or floppy (USB FDD often works well). See computer and/or motherboard manufacturer web sites for more information and for BIOS upgrades / tools. There is also other tools around google.
If you happen to have another computer with same motherboard or maybe identical configuration you can do BIOS hotswap to repair broken and/or inconsistent BIOS settings. To do this you need to take BIOS chip from working computer and place it to broken, then start computer. After BIOS is loaded you can remove chip while computer is running (be very very carefully) and put broken one in. After that either setup it correctly or reinstall/upgrade.