Q: Why does my iphone 6s not open after the screen slowly died down?

I was playing pokemon go, my battery was around 50%, i plugged it in the charger and the screen suddenly becomes grainy and dark. As I tried to exit the screen or turn the device off I realized that the touch screen and the buttons does not work. even hard reset didn't work. And so i just watched and waited, I noticed that as the minutes passed the screen goes darker and darker until the screen totally shuts down. I tried turning it back on, hard resetting, charging it for a few good hours using different chargers (the battery logo does not show) and even connecting it to 2 different computers to see if iTunes would recognize it but, to no avail. It has been 2 days now and my phone screen is still pitch black. Every now and then I still try to turn it on or hard reset but the phone just does not respond to anything. I remember updating the iOS to 9.3.4 about a day or two before this happened, I'm not sure if it has something to do with that because I did not drop my phone or anything like that before the incident. The iPhone was bought just this May of this year.