HELP! my computer wont turn on

Last night i came home from work and tried to turn on my computer but

when i pressed the power button nothing happened!

i looked at the back of the pc and the psu light was on but

the moment i pressed the power butto, the light went out.

i went to HP's site to see what to do and

i followed their steps and found out that when my motherboard is connected

to the psu the psu light goes off, and when my motherboard is NOT

connected the light stays on.

HOWEVER the fan for my psu is not working when i have it disconnected

from the motherboard so i am not sure as to if its the motherboard or

my psu that has a problem.

i dusted out my computer only about a month or two ago and my only

guess is that either my motherboard or my psu has burnt out

but im not sure as to which one is the burnt out hardware

so PLEASE tech gurus, HELP ME D:

if youre wondering how i wrote this post, i did it via my brother's laptop