All my address books have disappeared, including the master list. This is a catastrophe, because I have dozens of group mails as well as a collection of hundreds of separate addresses. What can have caused this, and please, how can I retrieve them?
I recommend you to look at the articles, and maybe ask in the official support forum:
https://support.mozilla.org/en-US/products/thunderbird
Also here:
http://forums.mozillazine.org/viewforum.php?f=39
You will need to know what version of TB, and whether it was recenly updated from 68.x to 78.x