If you have the urge to say suddenly my mail stopped downloading / working. It is more than likely not an issue with your mail client. Sometimes it is the mail provider changing connection settings so these need to be confirmed. Other times if it is following an update to Thunderbird, then it is more than likely the firewall component in your internet security suite / anti virus.

I suggest you look at the anti virus/firewall aspect.

Thanks for that but I turned the firewall off and it made no difference. I have since reset Thunderbird to it's previous state and emails flow again. Think I'll abandon all attempts to update

When you get a virus or other security problem because of unpatched software, just remember it was your choice.