I have just bought a 400D and had no problems initially in setting it up. I also bought a BG-E3 power grip, plugged in a charged battery and fitted it. NOw the camera won't power up at all, even without the power grip fitted...help!!!
a number of new generation devices suffer from DOA (or early death)
often stuff happens on the first day or shortly after
thats where warranty comes in handy
return to vendor and replacement usually is offered without question
otherwise run a multimeter over the batteries and see if they have discharged themselves or failed
Usually answered in minutes!