I have replaced the digitizers on ipad Air before with no problem but this time we had one with cracked LCD screen. Replaced the screen, but it stayed black. With the old cracked screen light was coming on and could see the image. As our distributor was only 20 miles away so I took the device to their office and tested with 4 more LCD screens. They all stayed black. They were also from the same manufacturer with same numbers at the back.
Decided to order from a different supplier. It worked. So obviously the problem was with this distributors screens. I contacted 5 different suppliers on ebay and aliexpress regarding this issue. They all said that they never had any problems with their screens. I bought from another supplier (Kamani, Slough, UK) on ebay who has sold about 73 so far ( £ 60 / unit ). Some people claim that these lcd screens are coded by Apple but it is not true. There are some screens in the market which just don't work at all.
During all this process I had one more problem which just appeared from nowhere. Although this was not originally the problem. When replaced the screen, ipad would not charge. However, it was charging fine before. It was making beeping sound after about 8 seconds and would also not switch on. Took the board out as it is just glued to the case. Checked all the connections and they all looked fine. Just left it for about 24 hours. Then pressed the power button and it switched on. Maybe it was holding some kind of charge which got released. It was fine after that.
Had some very painful few days but wanted to share my problems here so that it might help someone.