Q: Iphone 6 plus

Hello, firoz friend.  

Thank you for visiting Apple Support Communities.  

I understand that your iPhone will restart when watching videos.  I would start with forcing all open applications to close and restart the device.  Once this is done, test to see if you can play videos without the device restarting.  

Force an app to close in iOS

Restart or reset your iPhone, iPad, or iPod touch

If the issue persists, go through the steps in the link below.  

If your iOS device restarts, displays the Apple logo, or powers off while you're using it

Cheers, 

Jason H.