If you encounter this blank screen due to having replaced the screen while the iPad is on, it's most likely just a soft reset it needs - power + home button for 10 secs, and it should boot up normally..:)
I can't say I'd advise doing so, but from time to time the iPad comes on while working on it, and you can't shut it down with no screen on it - reboot, and it's good..
Also, if you screw the battery-screw out, and take a plastic tool and place between battery and logicboard, you should be able to lift the logicboard 1 or 2mm, which is enough for it to loose connection, and turn off..:)