This is more than likely the supposed security junk that gmail dreamed up. Go to gmail and read about 2 step verification and application specific passwords or turning on access for less secure applications.

Brianv13_2 said

When attempting to retrieve messages in Thunderbird from my gmail account the password is not recognized. There had been no problems until Friday. Using Thunderbird 31.3.0. on Windows 7 machine. I can sign in to gmail via the web. Error message says to log in using http://support.google.com/mail/accounts/bin/answer.p4?answer=78754(failure). This was attempted. Instructions followed. Same result. No email.

Turning on access to less secure applications was the answer.

Thank you