Hard-reset is required in the following situations:
When your handset becomes too slow.
When some of the apps do not function and you cannot remove them.
When there is some software or OS related problem which you cannot resolve.
When your handset is pattern locked. You have forgotten the pattern as well as the gmail account ID/password associated with that device.
There are 2 methods of doing a hard reset. First method is by doing it from the phone’s settings. While the second method is from the recovery menu.
Go to the settings option in the menu.
Search for the ‘Backup Ad Restore’ option and select it.
Now select the option ‘Factory Data Reset’
Now select the option ‘Reset Phone’.
The device may ask for the confirmation from your end by the message ‘Erase Everything’. Confirm and your gadget will start the process. It will restart now and you are done!
Switch off your Xperia Z. Its better if you remove your battery and insert it again.
Keeping the device being switched off, press and hold the Volume Up and Volume Down buttons. Keep these two buttons pressed and then press and hold the Power button.
Keeping these three buttons pressed will bring a menu on the screen. Navigate to the recovery option with Volume Up and Down buttons. Select the recovery option with the power button.
Now you will see an Android logo on the screen. Press and hold the volume up and volume down buttons, which will bring your device to the recovery menu.
The recovery menu will have many options in it. You will need to search for an option like Wipe data/Factory reset. Now press power button to select it.
The process will start and complete in less than a minute and your phone will restart automatically. Now you have successfully performed hard rest of your Sony Experia Z.