If you forgot your mail account that you previously used on this device, the only way to unlock your phone is to perform a factory reset. By doing this your password will be removed, but you will also lose all your data from the mobile phone. To make a factory reset, you first need to turn OFF the Android device. After you did this, try to press and hold on the Volume down button + Power key with your left hand. When the LED from the bottom of the page lights out, then you must press the Home key also. You need to keep the 3 keys pressed until the LG logo disappears. After that, your phone will perform the factory reset and you need to wait. When the wiping process is over, you should be able to use your device again.
