how do we restart an iphone 6s

You may restart an iPhone 6s by pressing and holding down the power button (sleep/wake button) for a few seconds until an option to shut down your phone appears on the screen. From there, you can swipe it right and wait till your phone shuts down. You may then turn on your iPhone again by pressing and holding down the power button again until an Apple Logo appears.

If your screen is stucked and unresponsive to touch, I would suggest force restarting. To do this, hold down your power button WHILE pressing your home button. Wait until your iPhone restarts and shows the Apple Logo.

If your iPhone is stucked on an Apple Logo, might I suggest reading "If your iPhone is stuck on the Apple logo - Apple Support (PH)"