After disassembly and assembly phone doesn't turn on

So many things can go wrong on a housing swap, especially for someone who is doing it for the first time.

First things first. If you connect your phone to an iTunes enabled computer, does iTunes recognize your device as connected? If it does, that means the phone booted but you can see anything on the screen. If you shine a bright flashlight on the screen, can you see a dim image? That would confirm a backlight issue.

If iTunes does not see your device, you could connect it to a USB Ammeter, you could use that to determine if the phone is really drawing current (normally ~1A) when it's supposed to be charging.

Finally, I would backtrack my steps and double check my work. Things to look for are torn flexes and incorrectly placed screws (long one where a short one would go - this can damage the logic board).

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