I know this is a pretty old post,, butI want to add my $.02.
I had 2 of these LG 31MU97-B’s sitting around for months from owners who hoped I could fix these. Both users gave up quickly, not wanting to spend more money on a problematic monitor. On an off chance that most things are simple I asked LG how to reset the board. Sitting around for months didn’t do it apparently. LG got back to me instructing me to hold the power button in for 30 seconds unplugged. Naw too easy and not logical. These were both dead as dead can be with not response. Boom! both started functioning perfectly again. What?? I’m still baffled.