Options/Security/Passwords/Saved Passwords, delete the old passwords and oauth tokens, then in Tools/Account Settings, set the authentication on the incoming and outgoing servers to OAuth2 if it isn't already, check that cookies are allowed in TB Options/Privacy, then restart TB and enter the new password in the OAuth browser window when prompted.

The problem was not solved by using the above. I noticed online that others have the same problem if they changed their gmail password. The last date that I have been able to use Thunderbird was 5-12-2020. I really like Thunderbird and hate to lose it. Margaret

If the account is POP, some users have found they have to enable access by less-secure apps in the Google account, even if the authentication is OAuth2, which is not the case with IMAP.

The account is IMAP.