Verify your credentials by logging in to your account via webmail. If you lost your password check Google's password reset options.

Since you do have a Google account you'll have to change your password on the Google website. You cannot change your Google account password using Thunderbird. You may have told Thunderbird to remember your password. Then you can look up and delete the remembered password via Options - Security.

In any case, Thunderbird will prompt you for a password when one is needed.

This will sound dumb, what do you mean by webmail? When I log into google I just bring up my email account. No PW asked for

I suspect your browser has stored it for you then.

Google accounts, and the associated googlemail accounts all have passwords.

Paaswords stored by Thunderbird can be found under Tools|Options|Security|Passwords→Saved Passwords

But with gmail you may also have to jump over additional security hurdles. By default a new gmail account set up in Thunderbird will elect to use OAuth2. If you don't use this you my have to set up an "application specific password" or permit "less secure applications".

If you're using IMAP it may be easiest just to delete the account in Thunderbird and set it up afresh.