^ + 1
Do you have a spare or borrow a video card to try it out?. Just as an extra step unplug the computer from the wall, push the power button to release any stored energy in components, and remove the cmos battery for a few seconds. Reinstall the battery, plug it back up and try again. Are you repeatedly tapping the delete key to get into bios? Are you sure you plugged in the 2nd power cord for the motherboard and the power cord(s) for the video? Sure you plugged in the monitor plug into the video card? Just asking and wanta make sure the basics are covered. Notice I didn't ask if you plugged the computer into the wall???
When you turn it off to reboot does it turn off in a sec or do you have to hold the off button down for 5/6 seconds before it turns off? If it's the latter then your computer is booting up, you just can't see it so that would be the card. Do you have fans and stuff that spin up when the powers turned on or does it do nothing? Keyboard and mouse lite up? Did you leave the screw for the video card out - maybe it is pulling the back of the video card slightly out of the back of the pci-e slot.