Welcome to the forum!
we have had many threads on this...because it is fairly common....and there are many possibilities
could be
power switch not hooked up to right pins on motherboard..double and TRIPLE check that
some boards/vid cards need that extra 4 pins connector for the aux12v from the power supply....so check for that
make sure the cpu fan is plugged into the right fan pins...some boards wont boot if they dont sense that cpu fan there
lots of times what happens is that the motherboard is shorting out or slightly touching the case somewhere....to find out what you do is take the board back out of the case and try booting it outside of the case...what you do is strip out all the extra cards like nics/sound cards/modems etc....get it down to just cpu/heatsink..one stick of ram..vid card......take th emotherboard out of the case and set it on the motherboard box on the antistatic packaging......
try booting it like that and see what happens.....of course the case switches wont be pluged in cuz the wires arent long enough..so what you do is carefully take a flatblade screwdriver (or hard drive jumper) and short the two pins together where the power switch WAS...just short them briefly and the board should start
of course other possibilities are "dead cpu" or maybe something was damaged by static etc
also you can check the cmos clear jumper.....right beside the battery on the mboard should be a little jumper....usually they come set to the "normal" position....but some come set on the "clear" or "reset" position and you have to move it over to the 'normal" position to boot
or if it is on the "normal" position now you can also try resetting the bios by moving that jumper to the 'clear" position for a few seconds...then move it back
JP