Are you sure that your memories and all the cables are connected properly? Try to disconnect-connect the power cable to the motherboard and stuff like that. Check so its ok...
This seems like a GFX card seating or slot problem or have you had your MOBO out recently ?
As you have checked the seating then it can only be a borked slot, Have you been trying to overclock your GFX card recently ?
Have you updated drivers recently ?
What was the initial cause of the prob & what had you been doing in the 2 days prior ?