I had issue where it seemed IPhone 6 Plus did not turn back on after replacing display. Also, was not being recognized via ITunes on PC. Was under impression phone was dead..
After reseating cables, and confirming screws were placed into proper location, I plugged phone into firewire to see if possibly battery issue. One hint to recognize phone was not dead was that the back of the phone would become warm after roughly 5 minutes. Second hint, when attempting to apply ribbon cable plate to display/Home button/Digitizer (don't recall the 4 th cable) location, I found a ribbon cable was covering one of the screw holes. Turned out, I had ribbon cables plugged in wrong order. As soon as I placed in proper order, phone powered on..
Hope this helps someone!