Q: My iPhone 6S randomly turns off

Hello Superfex,

It sounds like your phone is restarting on you randomly for some reason. This can be caused by either a software issue, or a hardware issue so to start I would quit all the running apps on your phone:

Force an app to close in iOS

1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.

 
When you have done that force the device to restart with the Home and the Sleep/Wake buttons, then test the issue again:

iOS: Turning off and on (restarting) and resetting
 
If the issue persists, backup your device to iTunes, then restore it as new and verify that it works. 
 
How to erase your iOS device and then set it up as a new device or restore it from backups

If it does, then restore your backup to either verify it still works and the software just needed reinstalled, or isolate the issue to the backup file itself. Thank you for using Apple Support Communities, let us know how that goes!