This may happen if the update of the add-on is stuck. Don't worry about your calendar data, it is left untouched and will be available again if you follow these steps to restore Lightning:
Go to the Advanced section of Thunderbird Preferences and click on Config Editor...
Copy or start typing extensions.installedDistroAddon.{e2fda1a4-762b-4020-b5ad-a41df1933103} into the search box
When extensions.installedDistroAddon... is listed in the preferences list, right-click on it and choose Reset from the context menu - the preference now isn't bold anymore and its value is empty
Close the editor, leave the Thunderbird options, open Thunderbird's add-ons manager and switch to the list of installed extensions
If Lightning is listed there, click on Remove to the right of the list item
Restart Thunderbird After Thunderbird has been relaunched, Lightning will be back and you will have access to your calendar data.