This is probably the infamous "dead strip." or Apples response to planned obsolence. I havent found any real solutions that worked for me. Some have reported having success with rolling back to an older iOS version and custom editting their settings tab to include the touch sensitivity option. This sorta worked for me...but it only seemed to relocate the dead strip to another section of the screen. But you probsbly cant even try that if you cant unlock your phone. You most likely have no other option other than replacing the entire Front panel even though the problem is only thr digitizer.