New screen does not work twice?

Hello,

I have a really weird problem with my iPhone 4S . I tried to fix the screen, but the screen I received was broken (would show nothing but backlight)

Today I received a replacement and I assembled the iPhone again with the replacement screen. Before screwing in all screws, I just connected the cables and tried to boot up the iPhone and see if it works. It booted, and the new screen worked just fine. Then I powered the Phone off again and screwed in the screws, and after that the iPhone would not show anything but backlight again!

I checked all connectors and they are fine.

Please help me with this problem, I don't know what to do anymore :(