Anti-virus software corrupting your mail file(s) is often the culprit. These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Thank you for all that. I have stopped email anti-virus checking. I possibly had too many messages in Inbox. I have only one Thunderbird profile dated two days ago but I used Windows "restore previous version" to restore the earliest available version but that made no difference. I was able to find the important messages on the email server and download them so panic averted. What else is missing I will, no doubt, discover later. Thanks again.