A Hard Reset reverts a phone to its original settings and a clean OS (Operating System) removing all data and information added to the cell phone by the mobile phone user.
In essence a Hard Reset erases everything from the phone and puts the phone back to like new condition as though it’s brand new right out of the box.
1. First thing you will have to do for a hard reset on Optimus Z is turning it OFF.
2. Then wait 10 seconds and press and hold volume down +home + power buttons at the same time.
3. After few seconds there should be LG logo on the screen, you can release the keys now.
4. Wait a moment and you will get to Recovery Mode screen.
5. Now using your volume buttons to navigate and power button to select move to next step.
6. Select wipe data/factory reset and wait untill your phone will do the rest.
7. Now your phone has finished hard reset process and will restart, congratulations!