Hello there.

You've removed all tasks from the calendar and calendar itself. You can restore this calendar ; Go to View> Calendar> Show Calendar panel. Hereby you can restore that calendar.

Recent issue If you deleted the data and you didn't close thunderbird. Press ctrl + Z for Win/Linux. Or press cmd + Z to undo the deleted task. But, did you use or had a backup at that time?

Is there a backup You've deleted all the tasks, okay. What calendar was there that you used there and erased in mistake? Google calendar or lightning? Are you using Windows or another one? You can find the "calendar data" folder in the profile folder when you close Thunderbird. Is it there? You can import these into the software if it s missing or deleted , as you wrote.

Local.sqlite If you saved these data or you do have a backup of those tasks / data. You can copy the local.sqlite from that backup device and put them back in the calendar data folder. This replaces the old empty space / file. Restart Thunderbird.

Thank you for your reply. I'm using Thunderbird on a mac (with macOS Tahoe). I was not backing up my calendar (not Google calendar, just native home calendar).

When I look under the profile, I see the folder "calendar-data." It contains cache.sqlite, deleted.sqlite, and local.sqlite.

Is there anything I can do with that?

Hello there

Looking at local.sqlite Can you see how much or how many kb the local.sqlite is ? Can you make a back up of this file and place it on the desktop? The most local.sqlite files are between 0kb and 64kb. You might look at the cache.sqlite file to find out if other files are there and the same files that has been erased? You might need tools for that to view the contents.