try reseating the video card and then if that doesn't work.....
try pulling out all the peripherals then installing just the proc/hsf, video card, and a hdd. Then try booting.
generally if I find a computer not booting the darn vid card is loose (again) Altho I guess that's what I get for not putting the hold-down screws on the cards in my boxes
BTW what mobo is it and who makes the BIOS