What version of Thunderbird are you using ?

How many profile name folder do you have in the 'Profiles' folder: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles folder 'profile name' folders by default are usually called 'xxxxxxxx.default' where the x's are letters and numbers.

click on each 'profile name' folder in turn to see contents. click on 'Mail' folder to see pop and Local Folders mail account. Click on mail account folder to see files. Make sure you have the View > Details option and 'Size' column header visible. This will help you to see what is contained in each profile name folder. The one with largest file sizes is likely to be the one with old emails.

In Thunderbird Help > TRoubleshooting Information bottom of Application Basics section...click on 'about:profiles' You should see a list of those profiles. look for the one with correct profile name below that profile, click on 'Set as default profile' click on 'Launch profile in a new browser' - it actually opens in a new window. Now close the original Thunderbird window.

re : When I rebooted and started Thunderbird, it asked me to create an account.

That is because for some reason, Thunderbird has created a new profile. New profile is empty so asks for you to create an account. This might be because it has lost track of old profile or another reason.

It is important to understand what version of Thunderbird you are using. Thunderbird now produces two versions for Windows OS.

Windows OS 32 bit computer can only use a Thunderbird windows32bit version.

Windows OS 64 bit computer can use a Thunderbird windows32bit version or a Thunderbird windows64bit version.

What are you using ? Is it a Thunderbird windows32bit version 68.1.2 ? Or is it a Thunderbird windows64bit version 68.1.2 ?

Did a thunderbird update occur or was it only a Windows OS update?

Running Thunderbird 60.9.0 (32-bit). When looking at what profile it is using via the troubleshooting it says that the profile is in C:\Users\TEMP\AppData\Roaming\Thunderbird\Profiles. Notice the "TEMP" rather than my user name "lvdsb". I found a profile in C:\Users\lvdsb\AppData etc that appears to have my old mail.

The TEMP profiles are the only ones displayed by the troubleshooting.

It's like it thinks the user name is TEMP

So this is just a matter of using the profile manager to use the correct profile.

SO using the about profiles, click to "create a new profile". Call it what you want.Choose a profile and select your old profile folder (the one in C:\Users\lvdsb\AppData\Roaming\Thunderbird\Profiles)

Out of interest, Is there anything at all in the C:\Users\TEMP\ location (I am trying to figure out just how such a location could be selected)

There were empty profiles in the TEMP path. Presumably, they were created when I started Thunderbird a few times and created an account. Thanks for the instructions about how to use an existing profile. That has worked and I am currently downloading hundreds of emails from the past week when it was broken  :)