If you're having trouble with your Gmail account in Thunderbird today, it might be due to Google now requiring OAuth 2.0 authentication.
Thunderbird SHOULD transition your account automatically, but if you're not able to connect, try going into your Gmail Account's Server Settings and manually changing "Authentication Method" to OAuth2 (it should NOT be "Normal.")
More information about this change can be found here:
https://support.mozilla.org/en-US/kb/automatic-conversion-google-mail-accounts-oauth20