If it's in Windows and it's the PC in the sig (no integrated graphics card) then I would reseat each memory stick or simply remove, clean pins with alcohol and install again. On Intel quad-channel motherboards it's common that sometimes one memory channel/stick is missing or is reporting in BIOS but is not available in Windows. Just my thoughts as it's hard to say exactly,
When first reading your post, I too was thinknig about on-board graphics but Woomack is right, you don't have it on this board.
A reseat of all RAM may be all you need and you could be happy again. Otherwise, a lengthy process of pulling all RAM and installing half. If they all work then remove those and install the other half. If they all work then you might have a bad slot since RAM has to be installed in certain configurations.
Using the MB manual, install half of your RAM in the slots that they call for. If they work, remove them and install the other half of your RAM in those same spots. If you are missing RAM, then test each stick one by one in the slot that the manual states. If you do not see your RAM as missing, then you may have a bad slot instead. I hope I'm getting the idea across.