I have run into a similar issue on iphone 4 models. In my case I found that if any connector gets disconnected(specifically the digitizer in my case) simply reconnecting it was not sufficient, but disconnecting then reconnecting the battery was also necessary, I am not sure why except that I know if the screen plug comes out(like happens so easily when testing with a screen that not uninstalled) it never comes back on until the phone reboots, and I suspect thats nearly impossible except by unplugging the battery, once the screen goes blank. I suspect there is some kind of initialization process that must be repeated before a screen will come back of after being disconnected.