You need work with gmail to reset a forgotten password.

Passwords are setup via webmail. This is were you update/modify any passwords. If you do not know the password, then there should be a 'forgotten password' or similar link which you need to use in order to get access to the webmail account.

Then you use that same password in Thunderbird.

If Thunderbird had previously stored that password, then you will find it here: Tools > Options > Security > Passwords tab click on 'Saved Passwords' button click on 'Show Passwords' button

If you need to reset the password because maybe you updated it via webmail, then you need to select both mailbox and smtp for that mail account and click on 'Remove' and then 'Close'. then click on OK. then restart thunderbird.

when you next Get Messages or Send an email, it will prompt for the password. Enter password and select the checkbox for Password Manager to remember the password. click on OK.