Cpu power problem

I have a Gigabyte GA-G31M-ES2L which was working fine at the beginning when i was setting everything up. later on i try to screw the motherboard to the case while the m/b was turned off, but eventually the motherboard stop working so and I figured out that when ever the cpu power is connected it doesn't come on anymore.... i've tried resseting the m/b, used different psu,case, and i took everything connected to the motherboard off.... it comes on when I unplug the cpu power but not when it's connected....

Once again, most common cause here is a mounting standoff in an incorrect position, causeing a short - carefully check that there is a mounting hole in the MOBO for every position you have a standoff installed at...