Thanks for being a part of the Apple Support Communities. From your post, I understand that you are having trouble with your iPhone 6s. Specifically, the display went black while you were using an app. You can hear sounds, like notifications, but are unable to get the display on. I'm happy to help!
Based on what you've described, I would recommend using the following steps from If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support :
If your screen is black or frozen
If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:
- On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
- On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
- On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.
If you continue to have trouble after the above steps, please review and follow instructions in the "If you need more help" section:
If you need more help
Contact Apple Support to set up service if you have any of these issues:
- You still see a black screen after charging, force restarting, or following the steps in this article.
- You see a black screen, but you can hear alerts, sounds, and vibrations.
- The screen remains black, but the device chirps when you unmute it and connect it to power.
- Your screen is on, but it doesn't respond when you tap, slide, or try other gestures.
- You can't force your device to restart because a button is broken or stuck.
- Your device is stuck on the Apple logo, displays a solid color, or isn't recognized in iTunes.
Have a great day!