I have a Samsung S8 phone that I have been using for maybe 2 years. Suddenly the PIN no longer works to unlock the phone so that I can use it. I don't get an error message, the screen just goes dark for a second or two, and then back to the "swipe to unlock" screen. I am sure I am using the correct PIN, it was working this morning.
I realized I should try a soft-reboot, so I looked up how to do it: hold down Power and Volume Down buttons at the same time for about 7 seconds until Samsung logo appears, then let go of both buttons. The next time I tried to use the PIN the phone worked as normal.