Built new computer, won't power up

>>    I pulled the motherboard out of the case, set it on the anti-static bag and tried running it there   <<   VERY BAD ! !  the bag is conductive, that is why it is antistatic. Put it on a cardboard, or non conductive material, evne carton is ok.
Now the possibilituies are :
-bad power supply, as said
-bad video card --> try another one
-bad ram     --> try another stick, or run with 1 stick (if you have more than one)
-bad mobo   --> have a warranty replacement

also, while it is out of the case, check that the mounting studs for the mobo match with the screw holes, and you got aonly as much standoffs as screw holes, to avoid shorting something

And disconnect the powerswitch from the board, use a small screwdriver to short the power pins to turn the PC on. Try it with only 1 RAM module, or none (you should get a post, maybe you need to connect the case speaker to hear it posting). Make sure the CPU is attached and seated properly, and locked down. Also make sure the heatsink is properly attached, if you used thermal transfer paste only a very small amount is sufficient.

I am using the 450W Antec power supply that came with the case, which is an Antec Sonata.

The LED next to the CMOS battery is lit when I turn on the power on the back of the power supply, but nothing changes when I press the case's power switch.

The 4-pin power connector is connected to the motherboard.

I will try booting with only one RAM module and by shorting the power switch pins on the motherboard.  Then I'll reseat the CPU and heatsink.  Thanks for the help so far!

No beeps? - may be:
- bad case switch
- bad power supply
- bad mobo
- wrong spec ram
- bad ram
- bad CPU

You -should- get POST beep codes with any of these problems:
- bad video card
- bad ram
- bad mobo
- bad CPU
- bad CPU fan (some systems)

try and check to see whether all pins leading to the front power buttins are plugged in  the right order in your motherboard (check the mothjerboard user manual) becaiuse i know from experience with asus motherboards of the pins are not connected properly then then it wont turn on and also my next question is does the Power light and HDD light on your sonata case come on if not then the connections from the frontpanel of your case have either come off  you may try taking the front face off the case just to see whether the wires are all plugged in.

It turns out this power supply has two 4-pin connectors in the exact same shape that fits into the motherboard's 4-pin power connector.  The power supply came installed in the case and one of the 4-pin connectors was hidden between the power supply and the case, so I used the only visible one.  After digging around I finally saw the other one, plugged it in, and everything booted up properly.  Thanks for the help everyone.  nobus, your description of the wire colors for the 4-pin connector finally led me to discovering the problem.