Chosen Solution
So , we tried to replace the iphone screen because its very cracked. At first it turned on but looked glitchy but the touch screen didnt want to register. After that it just wouldn’t turn on. We checked everything and nothing was damaged. Put the old screen back on but no dice, still nothing. I have it plugged into the computer and ive tried hard rests and dfu. Itunes recognizes the device and i’m doing a reset/ restore but it still won’t turn on. Was it broken somehow while trying to switch screens? how come it worked and then suddenly worse off without doing anything?
As Ben suggested, not disconnecting the battery first can cause the backlight circuit to fail. Seeing as how both your original and replacement screen do not work, the problems seems to be on the logic board. Do what you normally would to “turn on” your phone. The check your display with a flashlight. If you can see a very dim image on your screen, then it is the backlight circuit. This will require a micro-soldering repair.
