my iphone 6s unexpected shut down and not restarting?

Try to reset your device. This won't delete your data stored on this device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

If this does not help, restore your device to factory settings. Note that this will erase the data from your device.

Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support

If this issue persists take your device back to the Apple Store for further diagnostics.

Hi

There is an iPhone 6s Program for Unexpected Shutdown Issues.

The program covers iPhone 6s devices within a limited serial number range that were manufactured between September and October 2015.

Apple advises that, if you have experienced the issue, you should visit an Apple Retail Store or an Apple Authorised Service Provider to have your iPhone examined and the serial number checked to confirm your eligibility for a battery replacement, free of charge.

More information and instructions:

https://www.apple.com/support/iphone6s-unexpectedshutdown/