Problem logging into Gmail from Thunderbird
To simply reset the account, go to Tools > Account Settings. Choose the account, go to the bottom left of the screen (Account Actions) and click "Remove Account."
Then re-add the account from scratch.
If you are using the IMAP protocol, you will not have lost any emails.
Though to be safe, you can navigate to where the data is stored and back this up first (usually in /Users/[username]/Library/Thunderbird/Profiles/)
If pop3, do the same as above but you will have to put the backed up pop3 emails back when you are done.