Long time for resetting procedure in iPhone 6
Try holding down the Home Button and Power button for at least 10 seconds, once you see the Apple Logo (after the 10 seconds) let go.
If this doesn't work you might have to restore in DFU mode, otherwise try contacting Apple Support (if possible, to try to save you the trouble of a restore). If not, I can give instructions on Restoring in DFU mode.