Put it in download mode and hook it in to your computer. Use the lg upgrade tool or lg support tool to flash the update to your phone. This will get rid of root and everything, but you will be able to boot. You can get all that back
Download Mode
Hook your phone into the comuter and hold power and volume down
When the lg logo appears, release power and volume down and immediately hold volume up
Install the drivers for your phone on the computer
http://lg-support-tool.en.lo4d.com/download
There is the link for the update tool. Follow the steps and eventually you should be able to boot. Hope this was helpful! It happened to me once!