We already shared a tutorial on how to fix a the Windows Phone spinning gears issue. Here is another method specifically aimed at fixing the issue on Nokia Lumia Windows Phones using the official Nokia Software Updater tool.The Nokia Software Updater for Retail allows you to flash your brick/dead Lumia smartphone with official firmware. This tool offers a simple SW update and flash method especially targeted for Nokia Care centers.
[DISCLAIMER] This guide is not means for regular users, so please note that you are proceeding at your own risk. This website will not be responsible for any loss of data or device malfunction which may occur as a result of using this guide.
[NOTE] Flashing with Nokia Software Updater for Retail will erase all data and applications on your device.
[NOTE] The flashing process may void your warranty.
- Download Nokia Software Updater (NSU) for Retail version 4.1 and install it on your computer (Windows 7 and Windows 8 only)
- - Connect your Nokia Lumia Windows Phone to your computer, run NSU for Retail and select 'My device does not boot up'.
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- - Once you see the device connection screen, press and hold the Power + Volume Down buttons simultaneously until the phone vibrates.
- - The application should now detect your Windows Phone and prompt you to download and install the recovery ROM for your device.
- - Once downloaded, the flashing processes will automatically begin. Please make sure you do not unplug your cable until the process is complete.
Once the process is complete you should have a fully operational Nokia Lumia Windows Phone device.
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source - My Nokia Blog