ATX power problem

[Corporate Thug]

i swapped a few things out of my case: upgraded video card, sound card, and HD. when i plug in the ATX power connecter from the psu into the motherboard, the computer wont go on if it's in all the way (into the mobo). if it's not in all the way, the computer turns on, all fans spin, the drives open and close, but nothing appears on the screen, any suggestions? thanx