Black Screen, and no power after screen replacment

Hello, i am working on 2 different devices, both of which are giving me the same problem. When i went to repair the phone, i did exactly how it said in your guide, but when i was finished the screen wouldn't come on, so i thought that it was dead, so i plugged it into a computer and it wouldn't even charge and the computer did not recognize it. I have fixed these before successfully and i don't get why these two aren't working. So i continued to order 2 more screens for the devices, when i went to install these new screens, i plugged them in before i transferred the camera, home, and ear piece over, and they worked. But after i transferred everything over and put it back together it gave me the EXACT same issue, please help!