My LG G4 has done turned itself off a couple of times in the past month or so. Usually waiting or removing the battery would start it up again.
Last night it simply turned off and won't start back up.
I've tried everything suggested on this post, including the factory reset but sadly nothing has worked for me.
Incidentally, the only way I could get to do a factory reset was to remove the battery, plug the phone in, wait for the phone to say "oi, where's your battery", put the battery back in and then press the relevant button combinations.
The factory reset crashed out the first two attempts (to which I had to perform the battery removal/plug it in trick each time) but on the third it worked. I thought I was getting somewhere when the phone booted up to the LG setup screen but it then crashed there, too.
Since then, the phone has been bricked. Nothing (not even removing the battery) will get a response.
O2 are kindly sending a replacement, which is great as it's a gem of a phone.