I had a similar problem to you, where it boots and flashes the LG logo for a second before disappearing.
You should still be able to access recovery, it's just harder to time. As usual, hold power and volume down until the LG logo comes up (in that second), immediately let go and hold the power and volume down buttons again. It should proceed as normal and ask you to factory reset. Hit power button a few times, and then recovery should boot up.
If you installed TWRP before hand, which, I hope you did, go to the terminal command, and type
dd if=/dev/zero of=/dev/block/platform/msm_sdcc.1/by-name/fota
Triple check your typing. You can potentially brick your phone typing it wrong.
After hitting enter and letting it do it's stuff, just reboot like normal.
This method worked for me, it came from (and all credit goes to him):
I haven't tried the adb way, but should be the same.
Let us know how it goes.
Best of luck!