iPhone 6 screen is black, but can still hear notifications, use Siri, etc. Cannot restart it.

My phone suddenly turned off, and now it's on a black screen and it doesn't want to reset or turn on even when I try to do so. However, I can make calls only by using Siri. I can hear notifications, calls, etc. But if someone calls me or messages me, I can't answer because I only see a black screen and even if I guess where the answer button is at (to answer a phone call), it still doesn't work, get what I mean?

Please help!