I got the same problem I found out that the problem was occured by connector 3 of the screen. After i replaced the screen it worked back like it should be =D
the strabged thing is just, that u put the logic board in the other phone, tested it, it didn't worked, restored it, and then it did work. then replaced the logic board back, and then it didn't work. Strange phone i have :p
I already did that and both screen work. Now yesterday i gave it another try, and i took my phone, pressed in the right upper corner on the screen were the connecter is underneath, and then it worked. but only verry short, after that it didn't work anymore. then it should be the connector right??