Sometimes apps will become unresponsive on your smartphone and thereby prevent further use of device. If your iPhone won't respond to any input after an app crash or error, iOS offers a simple way to troubleshoot this issue.
This tutorial will teach you how to force quit apps on the iPhone 6S/6S Plus without the need to restart your device.
How To Close Unresponsive Apps on the iPhone 6S
Hold down the Power key until the Power slider appears. Release the Power key and then hold down the Home key for a couple of seconds. iOS will force quit all active apps and take you back to your home screen.
Warning: Holding down the [Power] + [Home] buttons at the same time will cause the device to restart (Soft Reset).