Dead phone after screen replacement

I've done a screen replacement on an iPhone 6 4.7 yesterday. I've done many of them already as I work in a busy phoneshop. The only difference with the other screen replacement is that I used a screen from our new furnisher which claim to be a "Genuine Replacement". The customer collected it and came back today as we give 3 months warranty with repairs. And the phone is completely dead. Tried battery pull, other battery, other screen, dfu, check if it appears on iTunes : Nothing. Does anyone have an idea what i could check ? Thanks.

Edit : The costumer came back and I heard that the phone had some charging problem as well. I've tried new charging port, no success.