I had this problem after re-assembling my phone after replacing a bad rear camera (thanks ebay and youtube!!!). turns out it was a loose ribbon cable from the display to the motherboard. I popped the thing back open and made sure all 3 ribbon cables that connect the front screen to the motherboard were secure. sure enough, one of them was loose and once i snapped it back into place it was fixed. if you look on youtube for iphone 5 screen replacement tutorial you will see the 3 ribbon cables i am talking about and how to access them. it does require a few tools to get inside the iphone. It is not rocket science but i could see how most people would be afraid to attempt opening up the iphone. my advice is this...hey its broken anyway...if you fix it you can put off buying a new phone longer, just take your time, do it slowly and carefully. Also a cell phone shop that does screen repair can do it, just know that the more naive you seem to them on the subject the more they will charge.