I've downloaded a new update (8.10.14219.341) for my Lumia 720. But when I try to install it, once the phone restarts, it just gets stuck at the spinning gear screen - there is no progress whatsoever. I have even tried restarting the phone twice but it goes to that screen again and gets stuck. Please help.
When the phone is in spinning gears mode with a progress bar, there is NO need for Wi-Fi at all. The phone hasn't even properly booted at that stage, let alone loaded the TCP/IP stack. It is merely, in your case, installing the package that was downloaded prior to the reboot.
If you have waited an hour and a half and the progress bar has not changed, then, unfortunately, it sounds like the package is maybe corrupted and can not be installed.
As Drowin suggests, your best bet is a Hard Reset - With the phone switched off, hold the Power and Volume-down (NOT Volume-up) keys at the same time, until you feel the phone vibrate. Then press Volume-down, you should see a large exclamation mark (!)...
At that point press Volume-up, Power, Volume-down. The phone will reboot and reset itself, after some time. See To reset an unresponsive phone
Once the phone has reset itself, you can then attempt to install the update again. Of course, the reset will wipe the data off your phone, but as you had backed up your phone that shouldn't be a problem...
Windows 10 Mobile Additions:
(This edit provided by an Anonymous User, without a quoted source for the information - so take it with a pinch of salt - and it may only apply to those users who are part of the Windows Insider Program1)
Doing this on Windows 10 Mobile, unfortunately will crash the update mechanism, as doing a hard reset removes you from the Windows Insider Program, and this will keep you from updating your device. In this situation, there are two methods to continue receiving insider updates:
Method 1 (Preference):
A way to fix this is simply re-downloading the Windows Insider application, and subscribing to the fast update. However, if an update is in progress, this will fall and updating will fail with errors. However, if an update fails, go into the Insider Program before hitting retry and it should work.
If all else fails, simply return to Windows 8.1 by using Windows Phone Recovery Tool, offered by Microsoft. Then download the Insider App and return to the latest Windows 10 Mobile Technical Preview.
It happened with me once. I've used the Windows Phone Recovery Tool.This application will reinstall the latest firmware available for your device and has a option specifically for non responsive devices, which is your case. Install the application and follow the instructions. You can find the application and more relevant informations about the process in the official Microsoft support page.
To get data as well as phone back, try this before resetting your phone:
- Disconnect the phone from the charger
- Remove the SIM and SD card
- Reboot your phone by pressing the Volume down and Power keys for 10 seconds
This should get the progress bar moving faster.