I'm having the same issue, but I'm not sure that is for the same reason.
I assembled the parts properly and on the first startup, the graphics card (ASUS Radeon 6970 2GB) had a single red LED lit up. The monitor of course could not find a signal. I tried a different cable, still nothing.
Then I took another graphics card (5770), put that in and it had three green lights at startup, but still no signal to the monitor.
Now I'm not sure where the problem lies, the graphics card, the motherboard, the cables or the monitor. The monitor is fairly new, no more than two years. The motherboard is a Gigabyte P67A-UD4-B3. I have a Corsair HX-650 PSU.