I have had a functioning system on wheeze using grub2. I did an upgrade to Jessie according to the debian docs. Now when I boot my system I simply get a black screen after the POST screen - there's no GRUB screen or any hint of it.I had a similar problem with a laptop. I resolved it (finally) with a BIOS upgrade.
Sadly this problem started AFTER I upgraded my BIOS. I have tried installing debian (AMD64) from scratch and am unable to boot from this either. I can't find any way to downgrade the BIOS and there is only one BIOS upgrade file on the Gigabyte website.
The only small piece of good news is that I've been able to restore my disk from a backup (prior to the debian upgrade).
Where should I go to from here?