My iphone 7 went blank and unresponsive I didn't dropped my iphone, no water damage, but suddenly when i was about to open Twitter, my screen froze and there were lines appeared and the lines started to cover all of the screen, and then the screen went blank and unresponsive. But the back light was on. The sound and vibration did not function. 
Again, i didnt drop my iphone. I tried to restart my iphone multiple times but nothing has changed. Then i did hard reset by itunes, also nothing change. I have been using my iphone 7 only for 5 months and it is still under warranty.
Can i know what is the possible reasons behind these problems? Was it motherboard or LCD damage? :(
I cant be sure without checking up the phone, but I fix iPhones and this sounds like a fault in the motherboard, sometimes this happens when you bend your phone a little by putting it in your back pocket or something. 

Anyways if you are still under Warranty I would suggest you to go directly to apple, most likely they will replace your phone at no cost.