Since this software runs on your computer, nobody but you can do anything to it.

All versions of Thunderbird have a matching version of Lightning. Sounds like you skipped the step of checking for upgrades to your add ons after you upgraded Thunderbird. This does not make the actual calendar disappear but all your events do.

Find the Lightning version that matches the Thunderbird version you are running and install it. https://addons.mozilla.org/en-US/thunderbird/addon/lightning/versions/

When I try to install w/the link given in you email I get a message that my machine does NOT recognize the extension. BTW...why do I suddenly need to manually update my calendar. It has always AUTOMATICALLY updated whenever I updated Thunderbird...also automatically.

How to install add-on. https://support.mozilla.org/en-US/kb/add-ons-and-extensions-faq

I have not installed v31 yet. I usually wait a week or two to see how it works out. I have never seen an upgrade that needs the addons upgraded not ask you after the upgrade. I doubt v31 is any different.

I have updates set to install automatically. Perhaps I need to rethink that...But trust me this uninstalled (or hid) w/out asking me.

Does the add-on Not appear in the add-on manager at all?

Tools menu (alt+T) > add-ons

If it is there it is hiding. I can't find it. When I try to download the suggestion in the previous post it downloads w/an extension I am unable to open.

You do not open the file. You install it using the instructions I supplied earlier.