My main 'Profile' in T-Bird start up not there

Does profile folder "Three" still exist in folder "Profiles" in .....\AppData\Roaming\Thunderbird\Profiles\ ? The way to verify: in Thunderbird go to Help > Troubleshoot Information > Profiles > about:profiles ....

Unfortunately no

Go back to Help > Troubleshoot Information > Profiles > about:profiles .... Click on Open Folder in one of the profiles: this will open the selected profile in your file explorer. From there go one step higher in the folder hierarchy to the folder "Profiles" and make a screen shot of its contents. Now go still one step higher to the folder Thunderbird which contains a file "profiles.ini" Open this file in a text editor and make a screen shot of its contents

OK got screen shot of the profiles.ini files - do you want to see that??

In order to help you I would like to see the screen shot with the contents of the "profiles.ini" file as well as a screen shot of the "Profiles" folder.

OK - Two screen shots attached

I cannot see a "Profile Three" neither in Profiles nor in profiles.ini You seem to use profile "new User" and/or "Profile Two". There is a "profile-one" in "Profiles" but it is not included in profiles.ini Actually when you start Thunderbird it opens the profile manager window. Please show a screen shot of the profile manager window (see image).

Attached. As you will see there are three Profiles, usually there were four, with Profile Three the lowest, which the one I used all of the time. It's just not there as of this morning.. (The one listed 'Profile - backup', was originally 'New User', but I currently am using that to at least to keep my own emails coming in - I have about 10 others that will have to be reinstalled!)

There is no Profile Three and I don't know where you could look for it, unless you have a recent backup copy of the complete folder "Thunderbird" with Profiles and profiles.ini To explore what is in "profile-one" I propose to create a new profile in the profile manager, then connect it via "Choose Folder" with the folder "profile-one" in Profiles and finally start Thunderbird in this profile.

What does " I have about 10 others that will have to be reinstalled!" mean? Ten other profiles or ten accounts?

This is all refers to my main desktop computer. The '10 others' refers to other email accounts that were all part of this Profile. I can re-establish all of them, but of course will have lost any previous emails. I do have my main email (ie the <mat@matirvine.com>) on on my laptop, so I could in theory transfer them all to the desktop. I have also lost the address book, but that will eventually recreate itself. Thanks