Out of curiosity: what version did you update Thunderbird from?

It should be safe to install and reinstall the add-on - any data should be left untouched. If you have doubts however, make sure to back up the calendar-data folder located in your profile folder first. Use the search field in the Add-ons Manager’s top right to find the most recent Lightning version.

If Lightning still doesn’t work, I suggest reinstalling the Thunderbird version you used before the update, as shown in the Update History in Options > Advanced > Update.

Does this help?

Hi, Yes reinstalled 45.5.0 and the Lightning app. All back to normal. Thanks,