Check with your email provider if they are having a problem.

Also see https://support.mozilla.org/kb/cannot-receive-messages

Everything working fine with ISP. Webmail through them is working okay.

If you recently updated and nothing is coming in or goingout, then it is possible your Firewall is blocking Thunderbird. Firewall may be part of your Anti-virus package.

Sometimes, after updates, the Firewall does not seem to know it is the same product. Close thunderbird Access Firewall settings and make sure Thunderbird is set to allow, On one occasion, I had to block and ok it, then access it again and set to Allow to get it to work. Then open Thunderbird.

Well I tried rebooting, but got nothing. But after having it shut down overnight it is now working again! Happy, but annoyed. I guess it was just having a bad day? Fingers crossed it isn't a common occurance. Thanks for your help guys!