This is the most reliable method for a completely unresponsive LG G3.
Once the phone is off, press and hold the key and the Power/Lock Key simultaneously. These are located on the back of the phone. hard reset lg g3 us cellular
How to Hard Reset LG G3 US Cellular Go to product viewer dialog for this item. (US990): Complete Step-by-Step Guide (specifically the US Cellular model This is the most reliable method for a