iPhone 6 stuck while back up version 9.3.5

Did you already try to force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again? You will not lose data by doing that.

If this does not work, make sure you have the latest version iTunes on your computer and connect to it in recovery mode, explained in this article:

If your iPhone doesn't respond or won't turn on - Apple Support