LG phone seemingly stuck on "Firmware Update" This may or may not be off-topic; feel free to point me to a better place to ask this.

My father brought home a cell phone he found in the woods, and I thought I'd charge it and turn it on to see if I can find out anything about the owner. (Needless to say I got no further than the lock screen.)  

The phone is now turned off and plugged in via USB (not connected to the computer, but directly to the electrical outlet). However, the screen now reads "Firmware Update - do not unplug the USB connection until the process is complete", with a progress bar stuck at zero. It also refuses to be turned on again.

What's the best course of action? That is, can I safely unplug the device? I would assume so since it's only connected to the electrical outlet, but who knows what's going on behind the scenes, and unplugging it may leave it in an inconsistent state. I'd also like to avoid connecting it directly to my computer.

I'm probably being overly cautious, but I have no experience with LG devices and it's someone else's to boot, so I'd like to make sure I'm not making any of the stupid mistakes I'm prone to making.  Better safe than sorry.

I can post a picture of the phone if needed &mdash; from the looks of it, it's some kind of LG G Flex.
Take out the battery while it is plugged up into a lap,computer or maybe the wall. Then leave it out for a few seconds,a battery with a red exclamation point should show up and that is when you insert the battery and turn it back on.