I can't guess if you knew to connect power to the video card, forgot that second power cable to the mainboard or tried it with just one stick of RAM and no case.
I just completed my build and went to test it out just to receive a "no signal" message from my monitor. Look at the debug display and it goes through various codes like 19, 4f, 9c, 79, and 99 before stopping at d6, "VGA could not be recognized." I tried using a different pci-e slot on my mobo and same thing happens. I'm not sure what the problem could be.
My specs are:
CPU: Intel i7 5820k
GPU: Sapphire Raedon R9 390x
PSU: EVGA 850G2
RAM: Corsair Vengeance LPX DDR4 (2x8) 16GB