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There are two reasons for this problem. Either the battery has failed or the phone is frozen. We can try to reset it.
Press and hold down the Power button for 20-40 seconds and this will soft reset it. If that didn't work, then connect it to your computer then hold down Power button for 20-40 seconds.
If that didn't work, then hold down Volume Down and Power for 20 seconds or hold down Volume Up and Power for 20 seconds.
If this has a removable battery, then remove the battery. Connect it to your computer and put the battery back in and turn it on by holding down Power button.
These are only ways to reset it. If it still not working, then either the battery is failed or the internal board has failed and you have to take it to LG to get the battery replaced or repair the phone.
If you have any questions or would like any additional information, please don't hesitate to ask.