First, I have no idea what may have happened. All I can do is make a guess that an update in past six months may have created a new profile and the folders you are missing are in a prior profile. You can investigate that by clicking help>troubleshootinginformation, scroll down to 'profile folder' and click 'about:profile' and view any unused folders to see if your folders are there. Another way is to go to c:\users\<yourid>\appdata\roaming\thunderbird\profiles and viewing the profiles in Windows File Explorer. If there are profiles there, look in the Mail\Local Folders folder.

Thanks for the response David. I have found what I think are the missing messages, and after a lot of fiddling around, I found a list of previous versions. I tried restoring the latest previous version (created a couple of days ago) of one particular file, which seemed to work OK, but the particular email I wanted was not restored (in fact, I'm pretty sure no emails were actually restored). So I guessed the emails that I had lost were deleted earlier. I last looked at the email in September, so I went back to a version created before September, but when I try and restore that version, I get an error "The backup operation could not start due to an internal error: Windows Backup encountered an error. Please try again (0x81000036)." The message actually came up twice. I have tried again several times, and for several previous versions, but without success (and always getting the same error message twice). I don't suppose it's the end of the world - I rarely look at old emails, except for this particular one (but the info I need, I will be able to get from the original sender, but it will take time getting them to respond); very occasionally, I do need to check up on other old emails, and I won't be able to do that in cases where I have archived to local folders (which I do to save space in my webmail). If I can't sort it, I'm going to have to dirch TB.

or even ditch!

I don't understand the backup error. If you have an old profile, why not just copy it?

As far as I can see I only have a single profile which is the default one. I'll take a photo and post it, but it will be later or more likely tomorrow as friends are just about to arrive

Too busy to do anything yesterday David, and I have tried to send a word document with screen prints that I took following your suggestions, but I am unable to do that (it only allows photos it seems). I’m afraid I am completely out of my depth here. I can’t see anything in the screen shots which relate to “old profiles”, and even if I could see an “old profile”, I wouldn’t have the first idea how to copy it, as you suggest. I think I am just going to give up, and hope that I never do really need my old emails. Thanks for trying anyway.

Ok, I understand. If you do get curious later, the message folders would be in an ...appdata\roaming\thunderbird\<profilename>\Mail folder or subfolder with a .msf suffix. Should you ever have trouble creating screenshots, there is a handy little free utility at bluefive dot pair dot com called snapshot. I also encourage you to evaluate whatever backup you were using that failed. I regularly see people posting here who are unable to restore backups created by proprietary software.