Frozen display

Hi there....I am a user of one of your product name iphone 6s. Today after using my device I turned the display off. Suddenly when I tried to use the device the power button was not responding and there was no display light. I was not able to wake up the screen by tapping the home button but the phone was on because I tried with another phone to call. The call was ringing but there was no display light and I also couldn't decline the call by tapping power button. In this case I desperately wanting to know that what's the real problem. Is there any system software problem? Or anything else? Hope your community will be honor to review my problem. Thanks.