The iPad does not like you turning it on or off, whilst disconnecting any of it's cables....
TRY NOT TO TURN IT ON OR OFF WHILST DOING THIS REPAIR...
If this does happen to you, as it happened to me, all you need to do is press the power button with the home button for 7 or so seconds for it to hard reset. (which may be a bit tricky if you have the digitizer off with a hanging home button, but you will figure how to do it after a few tries...
Also if there is no pic but is sound then you need to disconnect and connect the LCD and or digitizer connectors in properly pushed all the way in, along with the control board to logic board cable tab in a closed position or it won't function, even if it's connected in....