I have virtually the same problem on my 13month old LGRay/X190 started yesterday. In the night time the screen seems to be black on the left hand side, getting gradually getting lighter to a dark grey on the right hand side. I can hear it when it's receives messages but can't access the, I have tried plugging it in to my laptop but no success.
If you can hear the notifications go off and the red LED blinking, as well as the black screen "glowing" without any displays showing up then I'm thinking the problem might be a bad firmware. I'm trying to fix my wife's LG Stylo 2 "Black Screen of Death" ATM...
A common fault with many fones is that the LCD display contacts on the base make become loose due to oxidisation on the contacts and dislodgement. A fone shop can take the dsiplay out and clean both the mainboard contacts and the display and then reseat the fone to repair it. I did this yesterday with an S5 and the repair bill was $75 inc new gorilla glass cover ($20).
My point is that it is not always that the LCD screen is damaged ... which would normally cost you around $120 or so to fix if this was the case.
I have the Stylo 3 Plus - I have had it since June. Today was the 1st time the screen went black. First it froze - then I looked again and I thought it shut off. I tried numerous times to hit the power button on the back, under the camera lens, it would make like a vibrate sound, and the red light would flash on the screen side on the upper left. I plugged it in. Still nothing. I tried the volume keys and power button- nothing.
I have this little tool that came with my last Samsung phone- it makes it easy to remove the back cover. I simply removed the battery for maybe 2 mins. Then put it back in. Snapped the cover on it, hit the power button and it came back on, nothing was lost.
Maybe you tried this already?