It's really, really dead... or your charger and/or cable are bad - or you've got a hardware issue...
First - try a different cable... then, a different charging source... finally, if it still doesn't work, you may need to consult an Apple GeniusBar or other qualified hardware repair service provider
You mean I presume with the power button on top? If you can't and by rehooking it back up with the charging cord, Your power button might have gone out of wack.