James' answer was integral to solving the similar problem for me. Phone powered itself off and would not power back on. Tried all the Google troubleshooter steps to no avail. Talked to Google support and got no further. They transferred me to LG support to apparently learn about repair options. The LG staffer was a totally incompetent person who kept telling me to look things up in my settings on my phone.
Then I found helpful information on opening the phone on this site's tear down. I ordered a new battery and bought a Torx T5 driver. I took the back cover off, disconnected the battery, tested the power switch and finally removed the battery to prepare for replacement (the part had not yet arrived). Then I ran into this posting and with the battery disconnected, followed James' suggestions and the phone came to life. I then reconnected the battery and it charged and closed it up and it works great. I am left wondering if I could have followed James' steps without opening the phone and removing the battery and had the same result.