Why is my screen not working now that I have replaced it?

Recently I replaced a broken screen on an iPhone 6 with a screen and digitizer. While the screen was broken, it showed a black spot In the middle of the screen a little bigger than a quarter and quite a bit of discoloration. The user was no longer able to see the screen well enough to text but could still make and receive calls, however the phone would lock itself and hang up phone calls and redial the last call at random. Eventually the screen went black with just a couple lines showing. Upon replacing the screen with the screen and digitizer, I tested the screen to ensure that everything worked properly before completely reassembling the phone. The user got locked out of the phone while the screen was messed up, but after putting the new one on, the lock screen appeared normal. A few days later after returning the phone to the user the screen is black and shows two lines towards the left side of the screen. The user said they have not dropped the phone or damaged in anyway since it was returned but I’m having trouble figuring out what could possibly be wrong with it since it was in working order when I returned it to them, other than they needed to connect to iTunes in order to get back into the phone, which they never did. Any help would be greatly appreciated. Thank you!