Iphone 3gs Blank then Black screen

Replaced the glass , digitizer last month . worked graeat for a few days .n I got the white screen . the vibrate & sound functions worked . Tried complete retore twice , 2nd time as a new phone . Now all get is a black screen .

Phone is recognized by Itunes and have restored , few more times . opened back up , checked all connectors , seemed Ok . Removd motherboard , disconnected battery for 30 minutes . checked all connectors , still hear the sounds & is seen by Itunes , but still the black screen . Got it in DFU mode once &by guessing & counting & restoring . Still the same . Is this a bad LCD or is it a software problem ? Thanks