That actually happened to me yesterday. I did use a MF cloth to clean and since then it has worked ok. Prior to yesterday, I did notice it was not always opening and had to push the power button to turn off the screen and then turn back on for it to work.
So far today, it was worked flawlessly.
However, I will NEVER purchase another phone with facial recognition. That was the one think I really liked about my LG; the fingerprint sensor on the rear. Perfect location and never failed to open up the phone. If LG could have done something with that terrible UI and update on a decent basis; I would probably be still using it.