Did you just update to 45 If so, this will probably fix your problem.

1. On the toolbar then 
2. Select extensions and remove the Lightning (calendar) addon
3. Restart Thunderbird
4. Reinstall Lightning/calendar addon - if you need the functionality

Please tell us, does that help?

Note

1. If you can't get to Add-ons, then start Thunderbird in safe mode by holding the shift key while starting Thunderbird.
2. If you still can't get to Add-ons, simply reinstall Thunderbird from http://getthunderbird.com/

Thanks for a fast response Matt. Lightening cannot be installed on this machine because of a missing extension. Does this mean that I can never install the To Day Pane? I have 5 other computers on my LAN, and they all run the T-Bird just fine. (touching wood!) Anyhow, Matt, is there hope for this problem? Have a great day! George

One other thing, Matt, I just noticed that Lightening is not installed on my other machines, and yet the To Day Panes are visible. Go figure! George

when you went to add-ons. did you click on the extension on the left? I am thinking not because you can not be looking at the right think if you have a today pane. It is a part of lightning so it must be installed.

Yes! Absolutely. Sorry, but I'm not sure I understand your reply. I can assure you, kind sir, that lighting is installed on one computer that I own. The other 4 machines do not have lightening installed and yet the To Day Pane is quite visible.

This article clearly confirms what Matt told you. The today Pane is part of Lightning. You want a today Pane? Install Lightning. https://support.mozilla.org/en-US/kb/lightning-user-interface