problem

[vocab]

Ok Im in the middle of building my new comp and well I got the cpu/ram and plugged in the psu.When I did that I flipped the on switch and the system fans and the nb fan began to whirl.So I power Down and continue building and Now when I flip that power switch on the back of the psu all you hear is a quick power sound and nothing turns on.

What do you think is the problem

help plz

thanks