What device do you have, what have you tried? In short, no, a factory reset won't unroot your device. You need to flash the stock firmware for your variant using the lg flash tool. on d800, and maybe other variants, to get into dl mode, hold down volume down and plug USB into your device from your computer. You can also try volume up and plug in USB from pc.
Follow the steps in this thread to go back to stock.
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