iPhone 8 Plus will not accept correct password

I have not changed my password in more than two years, but suddenly today my iPhone 8 Plus will not accept my password. It was working fine all day, but I set it down for about an hour and when I picked it up again and tried to unlock it with my thumb print it a message appeared instructing me to enter my passcode. I have attempted the passcode, which I know is correct, several times but it will not accept it. I tried connecting the phone to my laptop to see if iTunes can access it but the phone again prompts me to enter my passcode. I do not use iCloud (and have no interest in ever doing so). Why has this happened? What can I do to unlock the phone? (Please do not tell me to restore it to the original settings)