Suddenly cannot connect
From MozillaZine Knowledge Base
If a working Thunderbird configuration suddenly can't connect to any of the mail servers see if you can still access your mail using webmail or with another email client such as Outlook Express. It can be usefull to have a account in Outlook Express configured to "leave a copy of messages on server" for this type of troubleshooting.
If the problem seems to be Thunderbird specific check your firewall settings. If your firewall (or security suite) is configured to automaticly upgrade it can lose the settings for some applications if it has a problem upgrading. For example, with ZoneAlarm sometimes the TrueVector service does not properly shut down before the upgrade is installed. If the firewall settings appear to be correct try doing a clean uninstall and re-install of the firewall. If that doesn't help try re-installing the prior version of the firewall, there might be a configuration specific bug in the newer version.
The same problem has also been seen with anti-virus programs. Try running temporarily with your anti-virus program disabled to see if its the culprit. Unfortunately some anti-virus programs are known to still run even when supposedly disabled so you may need to uninstall it and re-install it afterwards. Its recommended that you do NOT scan outgoing messages. Scanning outgoing messages can cause interoperability problems (especially if you use a secure connection) and if the recipient doesn't scan their incoming messages they have bigger problems to worry about than your message.
A good rule of thumb is to identify what was recently updated (or what changed) when Thunderbird suddenly stops working. This includes Windows Update installing a patch (though if thats the cause it normally causes problems for multiple applications).
You can enable POP3 or SMTP logging per these instructions and see if the log file is empty after trying to send or check for new mail. If its not empty then you made a tcp-ip connection to the mail server (i.e. its not a firewall problem) and your problem might be due to something changing your account settings. For example, they can get corrupted if you crashed while running Thunderbird