first off....
check your monitor connection
cable, power... etc
also check to see if you have a video card...
and if you do. make sure the AUX power is plugged in.
if you dont have a video card or have a video card that does not require an AUX power source than do a barebones boot by removing (unplugging power and data cables) all un-needed PCI cards (your vid card too if your mobo supports onboard video) cdrom drives and hard drives (including your floppy)
see if your computer will boot up now. If not, i would recommend trying a different PSU in your computer and or getting a multimeter to check it. I am sure there are FAQ's on how to do that on here..... somewhere.....