computer won't start

I got my new computer that I've been wanting for awhile. I put it all together and couldn't wait to set up a game. Thing is, it won't start. I push the power switch and the computer turns on for about a second, maybe half a second and then shuts off. I have everything connected properly and have checked it many times. I even unhooked everything except the power supply and the mobo and it does the same thing. It's not my devices because the ps should start up right? It's a generic ps that came with my case, 420w. Anyone know what the problem might be?
 
If you're talking about the heatsink to the processor, it is in firm and in the correct position. My neighbor came over and told me it probably is my ps. I am not sure.
 
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