As an addendum I have tried deleting Thunderbird from programs, then restart computer, then (new) download Thunderbird, then open and install Thunderbird = same outcome. Thunderbird will allow sending mail. Thunderbird will NOT download mail.

Is this for a pop mail account ? Perhaps there is a log jam causing the issue. Logon to webmail account via a browser and empty the 'Spam' folder In Thunderbird try 'Get Messages'

If still no download....in webmail account, delete the oldest not downloaded email from Inbox. In Thunderbird try 'Get Messages'

Yes it is a pop mail account. But I am sure it is not a log jam issue. In any event I have carried out some serious culling of mail in the Thunderbird account and also in the Outlook web mail account. No change. I think that there is something significant when Thunderbird is opened - it flashes a tab page similar to that when there is an update and the tab page describes the new features. This tab page flashes up for a brief moment, than the bar on the bottom states that Thunderbird is checking inbox, then that it is downloading 1 of 97, then it simply stops as if it has completed the download. I have been checking and deleting mail using the Outlook web page, but cannot download incoming mail in Thunderbird. I can still write and send mail through Thunderbird. I appreciate any suggestions and help.

I note that Thunderbird V 60.4.0 was issued on 20 December 2018. Is it significant that my last email downloaded into my Thunderbird account was on 20 December 2018. I assumed that if this is a bug there would be widespread problems and attention. Should I file a bug report?

I read the following from the Release Notes FIXED: Outlook import. Note: Mail, address book and settings need to be imported individually as "Import Everything" currently does not work. Does this comment relate to my problem?

Well I still believe in miracles. I deleted Thunderbird 60.4.0; and downloaded the Beta version 65, noting else had worked and I was in a three week IT frustration, but it worked.

Using Nortons is my guess. it seems to pop up frequently when folk start having difficulties like this. It also fixes it self over time if you are prepared to wait it out.

But I would think there are only a couple of possibilities other than anti virus. One is you cancelled a password request while the update was occurring. If it was that, installing TB60 again will work.

Matt - thanks for your response. I am an AVG user, not Norton, but I don't think that the firewall was an issue as I checked it. I have deleted Thunderbird & reinstalled umpteen times since 20 December & culled email - all without getting Thunderbird to download mail (still sent mail). But I am happy to announce that Thunderbird appears to be operational. I downloaded the Beta version 65.0b1. At first I thought no improvement, but after a reboot, miracles occur, and Thunderbird now downloads mail. When Thunderbird is started it still flashes a tab page briefly, but now it then does download mail. Thanks for your suggestions