LG Optimus F3Q hard reset
Using the Optimus LG F3Q / LG D520 hard resetIf your phone needs to be restored to its original condition, use a Hard Reset to initialize your phone.
Warning! If you perform a Hard Reset, all user applications and user data will be deleted. Please remember to back up any important data before performing a Hard Reset.
1 Turn the phone off.
2 Press and hold the Power/Lock Key + Volume Down Key at the same time.
3 Release the keys when the FACTORY HARD RESET screen appears.
4 Press the Power/Lock Key to continue or press any other key to cancel.
5 Press the Power/Lock Key again to confirm. Your phone will be restored to its factory settings. To cancel, press any other key.

or
You can use LG Mobile Support tool to repair your LG cell phone, after download select menu Customer Support > Recovery Phone (D520)
Go to Support > Mobile Phone Support > Select the Model D520
Using LG Optimus F3Q Safe Mode
To recover your phone when malfunctioning.1 Turn off your phone and reboot. When you see the T-Mobile animation
press and hold the Volume Down Key until the phone reboots. Your main
screen will then be displayed with the words “Safe mode” in the lower left
corner.
2 From the Home screen, tap the Menu Key and tap System settings >
Apps.
3 Tap one of the available tabs at the top of the screen and select the application
you wish to uninstall.
4 Tap Uninstall and OK to confirm.
5 After uninstalling the application, reboot your phone.