Screen no working after replacement

Hello! Can you please help me with my problem. I am fixing mobiles for my friends just for a hobby here in Finland, and now I got a problem, again.. My project is a Nokia 625, and I replaced lcd (the whole assembly) and everything went quite well, no any problems before I tested it. There is those 3 buttons you can see when the phone is closed also, and when I push them, the mobile vibrates like it should. But the screen is all black, nothing over there. Only thing I got to work is those 3 buttons. Then I tried to change back the old screen, to make sure the new part is just not bad one, but I got excatly same problem with the old screen. So now my friends mobile doesn´t work and I was just trying to help. Can you please help me, what I should do now? -Juho, Finland