wont power on as in?
wont post, or just does nothing when you hit the power button?
If it does nothing when you hit the power button, have you actually got power? is there a switch on the rear of the psu? flick it. is the power button itself at fault? try manually shorting the pins together with a screw driver
if it does power on and you get fan spinning but nothing else, check your 4/8 pin pci-e power cable is plugged in.
do you have a speaker attached to the mobo? does it give a post beep?
when building a new pc ALWAYS build it outside of the case first, i do it on top of the mobo box, jsut plug everything in or the minimal needed to make a computer work and turn it on, by manually shorting the + and - pins together.