I just had this issue, saw on the lock screen that the find my mobile app was updated, then it wouldn’t recognise my PIN, stressing for sure, so I did a bit of digging… First of all find out which model you have and find out how to restart your tablet by holding certain buttons, I have a SM-T510, my restart was holding the power and volume down buttons until the screen went black, logo appeared. Tablet restarted and my PIN worked again!