Have you enabled Gmail to work with Thunderbird? There's a useful article here: http://kb.mozillazine.org/Using_Gmail_with_Thunderbird_and_Mozilla_Suite In particular, make sure that you have set up Gmail to use POP or IMAP (as you prefer); you may also have to enable it to use 'less secure clients' (read the section some way down the page). There's also a Gmail troubleshooter here: https://support.google.com/mail/topic/3397501?hl=en&ref_topic=3398031

Thunderbird 38 has been updated to work with Gmail's two-factor authentication, so you no longer need to downgrade the security in Gmail settings. This article has more on that: Thunderbird and Gmail.

Because Thunderbird is storing your Gmail password and anyone with access to your system can then use your account, you might have chosen to protect your Gmail password with a Master Password. In that case, Thunderbird will request your Master Password before checking mail the first time. Could that be the problem -- Thunderbird asking for a different password? More info: Protect your Thunderbird passwords with a Primary Password.

Thunderbird 38 has been updated to work with Gmail's two-factor authentication, so you no longer need to downgrade the security in Gmail settings.

As of v38 Thunderbird supports OAuth2 authentication, which eliminates the need to allow 'less secure apps'. Thunderbird does not support Google 2-step authentication.

Because Thunderbird is storing your Gmail password

With OAuth2 authentication Thunderbird does not store the Gmail password but an authentication token. That's the whole point of using OAuth2. The master password is still required though if one is set.

Note, OAuth2 only works for IMAP and SMTP, not POP.

Thanks for your replies but it still doesn't solve my problem. I have been using the same set up for years so why suddenly won't work. I can access my gmail from the browser but not from Thunderbird. I checked my passwords in Thunderbird and it was correct but still doesn't work. I am not using the master password or the 2-step-authentication. Hope there is an answer somewhere.

Thanks again

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the Copy text to clipboard button and paste the information into your reply.