iPhone 6plus freezes and unresponsive
I haven't experienced this, but I would highly recommend reinstalling the latest iOS (iOS 8.2) onto it, by using a computer with iTunes and hooking your phone to the computer with the USB data cable.

In iTunes, it should detect an update available and ask if you would like to update.  Go ahead and update it that way.  If that doesn't work, manually download the iOS 8.2 firmware update for your device from IPSW Downloads and then, with your phone plugged into the computer via the USB Data cable, and iTunes open, hold down the shift or control key while clicking on "Update" on iTunes.  This will open a file selection dialog/window, then just select the file you downloaded (iOS 8.2 Firmware file that you downloaded from ipsw.me), and it will flash your phone with the latest version.

Hopefully this will take care of the issue.

If not, you may have to take it to an Apple store and have them fix it/replace it.