Chosen Solution
I wanted to fix my friend his broken screen so I replaced it and it worked fine, until I had to push a little harder to because it would not close at the top. After opening it up again the replacement screen won’t turn on anymore… I tried putting on the old screen and that worked until now, it won’t turn on anymore either… The area right under the lcd cable socket is getting pretty warm when I’m turning it on. Is there any way I can fix this?
Sounds like a bad connection or a short circuit.
Is it recognised in iTunes, does it vibrate when you flick the mute switch? If you think the phone is on but the display is hardly visible (shine a torch into the display in a dark room) you may have blown the back light. That you need a board level repair. Could you have put the wrong screws in the connector shield? If so this could have damaged the PCB. If non of the above, clean the connectors with isopropyl, check non of the pins are damaged and make sure the screen flex seat into the flex connector with a nice click.
