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This question already has an answer here:

My Lumia 520 got an update to Windows 10 Mobile. I started upgrading but then I remembered that i had low disk space. Now whenever I start the phone, I see moving gears on the screen but after some time the phone restarts and it happens again. The battery is fully charged but it happens all the time.

marked as duplicate by Indrek, row1 Jan 11 '16 at 5:48

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try soft reset, please try the following: Hold the two buttons (power+volumeDown) until it vibrates, release it and it will vibrate 3 times and restart. If there is enough battery the phone should restart normally. If not, please follow the instructions below:

  1. Download the NSU (version 4.0.1 for retail). Install the suite to your PC (Windows 7 & 8 only). http://nds2.fds-fire.nokia.com/fds_fire/1310/0909/7297834135/NokiaSoftwareUpdaterForRetail_4.1.0.exe
  2. Simply plug in your Windows Phone to the PC, fire up the software suite and choose "My device doesn't boot up."
  3. Once the device connection screen shows up, press and hold the power + volume down buttons simultaneously until the phone vibrates.
  4. The software will then detect your Lumia Windows Phone and prompt you to install 1.2GB worth of recovery.

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