try using something besides HDMI, even put a temp. video card in if you have one, just to boot. Then get into the BIOS and change the default video output to HDMI.
Exactly what I did,
I connected an external card, got it working, got to the bios, selected the onboard as primary. Removed the external card, and no video again!
plug (external video card) in again, and in the bios set to use "onbaord if no primary" video card, and set the primary output to HDMI
I did the first part, there's no option of selecting the HDMI as primary, I selected the intel ING as primary, but no good.