First time I've experienced this after using thunderbird for at least a year. A good search for very helpful and it came up with these two links which is the basis of my post:
As a reminder for me this is how I fix my problem.
1. Head to https://www.google.com/accounts/DisplayUnlockCaptcha
2. Successfully unlock my account with that link
3. Login to gmail from web interface and go to Gmail » Settings » Browser Connection > Always use https
3. If using googleapps. it seems this can happen when you do not enable ssl(enforcing ssl for domains) at domain settings-->General - SSL (check enable ssl)
Apparently no option was selected in my case. That is probably the problem. Dunno how it became unselected but these are one of those internet glitches that happen.