It looks as though your profile folder may have become corrupted. As a first step make a backup copy of your current profile folder (all of it). Keep it somewhere safe. Then read this article to understand the structure of folders within your profile: http://kb.mozillazine.org/Profile_folder_-_Thunderbird Look in your profile for a 'calendar-data' folder. If it isn't empty then you probably haven't lost your data.

Thanks - I backed up my profile folder, and found that my 'calendar-data' folder has 2 files (local.zqlite, deleted.sqlite) plus a backup folder with 5 files. Should I try installing Lightning again? If so, will my calendar items reappear automatically or do I need to do something else? Do I need to reinstall TB?

If I were in your place I would try installing Lightning again, it should pick up any calendar data stored in your profile. However no one here can guarantee that it will work, so you do it at your own risk. If it doesn't work and it ruins your current installation of Thunderbird, then you can uninstall Thunderbird, restart your computer and then install Thunderbird + Lightning afresh. So long as you have your current profile folder backed up safely you can then copy all your current data into the new profile. Instructions are given here: https://support.mozilla.org/en-US/kb/profiles-tb 'restoring a profile backup' is the relevant bit, but I'd advise that you read the whole article so you understand what's involved.

I tried what you recommended. When I installed TB, it automatically found my old profile, so I didn't have to restore it, and all my messages and settings reappeared. Before I attempted to install Lightning, I found errors in the error console.

Continued in next answer...

I just realized Lighting is back, along with all my calendar items ( I hadn't yet clicked on the "switch to calendar tab" icon in the top right corner). I still have the items below in my error console. Should I be worried about these, if everything else seems to be working normally?

Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/chrome.manifest'.

Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Thunderbird/extensions/%7B972ce4c6-7e08-4474-a285-3208198ce6fd%7D/chrome.manifest'.

Warning: Use of Mutation Events is deprecated. Use MutationObserver instead. Source File: chrome://calendar/content/widgets/calendar-widgets.xml Line: 496

Error: Please do not load stuff in the multimessage browser directly, use the SummaryFrameManager instead. Source File: resource://gre/modules/summaryFrameManager.js Line: 84

Error: uncaught exception: 2147746065

None of those errors are a concern. they stem or the first few do from changes Mozilla made to the way the default theme works.