I just built my new P5Q Pro today.  It will not start.  When I plug it in the green light on the MB comes on but when I push the power button, the fans sort of stutter, maybe try to turn, but then stop.
 
There is no sound just nothing.  A couple of times the num lock, cap lock lights flashed on the key board.
 
Here is what i have
 
P5Q Pro
Q6600
Corsair VX 550W PSU
1 ASUS EAH 8450
Corsair XMS2 DHX 6400 4 g (2x2) in the two yellow slots
WD6400AAKS HD
 
I checked all the connections but there was no change.
 
What did I do wrong

Try putting your ram into different banks.

Try plugging in a different psu as well as switching out the ram for the slots. Also depending on what video card/cards you are using, you could be overloading teh 550 watt PSU, unlikely but possible, unplug HDD's and Cd drivers and see if it posts. Make sure your cpu and heatsink are properly seated, it could be and emergency shutdown because of temperature sky rocketing. Hope this helps man.

If you meant 1 Asus 4850, you might not have enough power.

Sorry about that, it is a 4850.  
 
I can see that the case fan might not keep running if the temps have not had a chance to increase but shouldn't the CPU fan start as soon as you hit the start button and keep running?