How to fix a Phone that is stuck in a (quick) reboot loop
- Remove the battery from device (if you can't turn it off, just continue on to the next steps)
- Don't connect the device to a PC yet
- Launch Windows Device Recovery Tool (https://www.microsoft.com/en-us/mobile/support/faq/?action=singleTopic&topic=FA142987)
- Select that "Your Device cannot be Discovered"
- Select the device type (e.g. Lumia Device for Microsoft built phones); it should be searching for your device...
- Install the battery and plug your phone in to the PC
- While pressing the Volume Down Button, turn on the phone (if it is already turned on, let it reboot while your holding the button)
- The device should be found; you can release the Volume Down button
- Download and then Install the image
- Note, I've seen the UI freeze during the download -- it's working, just give it 5-10min.
- Go though Setup again...