Big Sur upgrade ...Now I cant open passwords or unlock preferences because it does not recognise my password. GRRRRRRRR!

I had the same problem. What I did to fix it was reset the SMC by doing the following.

1. Shut down your Mac.
2. Press and hold the power button for 10 seconds, then release the button.
3. Wait a few seconds, then press the power button to turn on your Mac.

https://support.apple.com/en-us/HT201295

Hello, I have the same problem on my MacBook Pro 16 . Installing the latest Big Sur update went without problems. When I turned on the computer this morning my password was no longer accepted.

How can I log in now?

How to Change of Reset Password

I have now restored the password via the iCloud and the keychain via the Timemaschin backup. Now I can use the Mac again.

Thank You!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!