If thunderbird was updated recently then it is possible your Firewall is blocking Thunderbird. Firewall may be part of your Anti-virus package. Do not switch Firewall off. Access Firewall and set Thunderbird to be allowed. I have Norton and whilst it has been behaving recently, in the past, I had to Close Thunderbird, access Norton firewall, set to block thunderbird, ok it, access again and set Thunderbird to allow and ok it. Restarted thunderbird and all was up and running again.