ipad: black screen when battery has charge

Some months ago, somebody walked on my friend's ipad 2 (model A1395 16GB) and broke the digitizer. The ipad was working with the touchscreen broken. It was then left untouhed for 2 or 3 months. My friend learned that I know a bit about electronics and computer and was capable and willing to help do a straight digitizer swap. She then bought the kit online and I have replaced the digitizer for her.

The problem now is this:

- when the battery is empty and I plug the ipad in the mains and press the home button, the screen shows for a few seconds, in the middle on a blue background, the empty battery with 3 red lines at the bottom. So the screen is woring, OK?

- wrong! When I come back after having waited for the battery to charge (wether it is 30 minutes, 2 hours or 24 hours), pressing the home the first time give me a kind of explosion or passing wave of light on the screen, lasting less than a second, then the screen stays black. No matter how many times I press the home button afterwards, the backlights light up but the screen remains black.

- after reading a few forums, I have tried the "spanking" method, or the way I do it it is the "light tapping" method, getting progressively stronger but always controlled (it's not my ipad). Of all the sisty or seventy taps I must have made at different times, I got a result only once: for 1 or 2 seconds I saw a screen (I think blue but can't be certain) with a turning circle in the middle. I think it was a circle made of dashes but I can't be sure, it was so brief! It was definitely animated though!

I don't screen the screen is broken, because it displays perfectly the empty battery logo.

What could happens that means the screen does not display when the battery is charged? Has anybody got any ideas?