No display on iPhone 6. Able to receive calls but blank display. Tried hard boot to no effect.

IPhone 6 display suddenly went complete blank. I am receiving calls, messages etc but the display isn't there. went to Apple Store. They have sent gor screening. Have assured me of a replacement aftwr the feedback from screening Center as the phone is under warranty period. inwould like to know why such a problem occur since my phone wasn't subjected to any kind of abuse. Is this problem common with iPhone 6? I have a iPhone 4S purchaed in 2012. Till date there is not a single problem on this phone Barring reduced battery life.