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Thunderbird suddenly won't work with Gmail - Login to server failed - SOLVED by unlocking gmail captcha

Been using Thunderbird with no problem for several weeks.
Now I am suddenly getting this error when starting Thunderbird and attempting to 'Get' mail:

Login to server imap.googlemail.com failed.
Retry / Enter New Password / Cancel

Then it gives this message:
Enter your password for sridharma108@gmail.com@imap.googlemail.com:

I have tried several times to put the password in manually, and have been very careful to put the right password. At the bottom of the screen and in 'Activity Manager' a message comes up that reads:

Alert from account sridharma108@gmail.com: Web login required: http://mail.google.com/support/bin/an... (Failure)

Gmail with the same account works fine through the web browser. So it's not the password or Gmail / the account in question itself.
It's only happening with Gmail accounts on Thunderbird.
Hotmail is working fine through Thunderbird.
Thunderbird is given an exception through Windows Firewall.
I have tried Disabling the firewall completely and that does not solve the problem. (Have since restarted the firewall)
I have No other firewalls and no other spyware etc. running.
Using Thunderbird 3.1.6
Using the latest Windows Vista
I am Not using a proxy of any sort.
I have No extensions, no add-ons, etc. installed.
Server is: imap.googlemail.com
Port is: 993
Connection Security: SSL/TLS
Authentication Method: Normal Password

This just started happening suddenly for no apparent reason.
I have not done anything unusual with the system nor changed anything nor added anything recently.
Is this some kind of known bug that just came up?
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