PC won't power up with ATX 4-pin connected to motherboard

hey guys, maybe someone can help me with this problem, my pc has been running fine for awhile, i was encoding a movie overnight, woke up and the pc is off, didnt pay much mind, get home from work and it wont power on. Long story short, i snooped around, unplugged the 4 pin from motherboard and she starts up!, plug it back in and when i press power button, she is dead again, any ideas? need a new MB??
 
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