If the phone is recognized by itunes but doesn't appear to be booting up, it is possible that the phone is booting but has no display or no backlight because the traces on the board below where the screws go to hold the plate over the LCD connectors. If you mix those up and stick the 1.7mm screw in the hole that a 1.3mm screw belongs in you do what's been appropriately named "long screwed" your phone. google long screw damage iphone 6.
If the phone is not booting at all, I still suspect the board is damaged somewhere. Repairing a logic board is beyond most DIY repair-ers