If you are experiencing no backlight after a screen replacement you have a short in the motherboard. This happens when you replace a screen with the iPad powered on. You will need to replace the filter and chip ic on the motherboard.
http://www.ebay.com/itm/141006779635?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1558.l2649
I fixed my iPad 2 with these parts but it was pretty difficult... you gotta have a small soldering tip and steady hands. I also used a microscope. Make sure your iPads are powered off before a screen replacement!