I can vouch for that motherboard, I'm using the same one, when you install a GPU it disables the onboard. So, if your not getting signal from the onboard video with the GPU removed, its probably the motherboard. Since the monitor works elsewhere, though I had trouble getting the onboard video to work on this rig. Never really tried to tell you the truth, I'm using an XFS nVidia 8800GS 384MB 192 PCI-E x16 2.0 bit GPU. Ensure your GPU is removed when you test the motherboard's graphics output. Otherwise the onboard WILL NOT work. The onboard video is tied onto the PCI-E bus, and is therefor, disabled when you use a PCI-E x16 GPU.
I don't know your full setup, so it could be a number of things, but even if the computer is off and unplugged the monitor should know its plugged into something. Maybe, on an off chance, the monitor isn't compatible with your hardware? Try another monitor with your system, and tell me, does the system turn on? do the fans spin up? LED's light up? Hard drive(s) spin? Or does it do nothing at all? I've ran into a few monitors that simply REFUSE to work with certain hardware. Odd, but plausible, though the reason as to why they don't work is a mystery to me. Only thing I know to tell you is try another monitor if everything lights up and spins.