Update to iOS 8.0.1 creates problems for Apple iPhone 6 and Apple iPhone 6 Plus
First, you will need to turn off Find My iPhone. Go to ipswdownloader.com and download the proper version of iOS 8 that you will need. Make sure you have the latest version of iTunes running. Launch iTunes.com on your desktop computer. Take your Lightning to USB cable and connect your iPhone to iTunes. Hold down Control (PC) or Option (Mac) and click on "Check for Update". Choose the iOS 8 firmware file you selected from ipswdownloader and let nature take its course.
Once you have your iPhone 6 up and running, don't forget to turn Find My iPhone back on. If this method isn't working, you can try a full restore, or wait for Apple to send out a fix.
source: iMore via TheVerge