For next time... deleting a mail account in Thunderbird will not fix anything. But it will leave artifacts on your hard dislk of the old account that makes fixing later created account more difficult.

When your ISP suggests deleting and re adding it is time to hang up. They have no idea what they are talking about. Having said that, I might not either.

You standard nothing changed, it just stopped is almost always something to do with an anti virus program or it's firewall component.

To diagnose problems with Thunderbird, try the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thank you for the suggestions. I will try them and get back to you.

Right for what it is worth. I got the 'Server has disconnected. The server may have gone down or there may be a network problem' and it was a pain. I'm with 1&1 hosting, running win10 and latest version thunderbird. I could see my settings on 1&1, compared with thunderbird, had changed and bit the bullet and removed my mail account and re-added it again. Thunderbird picked up the new settings and had kept all my folders, mail etc. So I'm now back up and running Jim

Matt, disabling the add-ons worked perfectly. Thanks for the suggestion.