Screen won't turn on iPhone 4

Hello,

Just replaced the screen of a friend's iPhone 4 because it had a crack and didn't turn on.

I installed everything, then turned the iPhone on.

=> Nothing on the screen and touch didn't work neither.

The silent switch worked (phone vibrates) and I can hear the lock sound when locking the iPhone. Once connected to iTunes the phone can be synced too, no DFU mode or whatever it can be.

Then I tried connecting another new screen, the same problem.

Other detail: when the screen is supposed to be off and I press the lock or home button it VERY shortly blinks a white color on the full screen and then nothing.

Is there a way to fix this?

Thanks in advance!