Hey all.
'''Unfortunately, In many cases this is due to damage unknowingly caused to logic board while putting the phone back together.'''
Some screws are very slightly (VERY slightly) longer than others and placing them in the wrong hole causes them to screw right into the logic board and through the traces (electrical connections on the board).
If this is the case you have three options:
1) New phone
2) New Logic board
3) Repair by someone very talented.
You'll have to hunt around for pricing etc.