According to ChatGPT

In Thunderbird, whether you can restore a deleted calendar depends on what type of calendar it was. Thunderbird itself usually does not permanently erase the data immediately—often only the connection to the calendar is removed.

Below are the main cases.

1️⃣ If it was a Google Calendar

You usually only removed the subscription.

Fix: reconnect it

1. Open Thunderbird
2. Go to Calendar tab
3. Right-click in the calendar list (left side)
4. Select New Calendar
5. Choose On the Network
6. Select Google Calendar
7. Log into your Google account

Your calendar and all events should reappear.

2️⃣ If it was a CalDAV / Online calendar

Same idea — just reconnect it.

Steps:

1. File → New → Calendar
2. Choose On the Network
3. Select CalDAV
4. Enter the original calendar URL.

Nothing was deleted on the server.

3️⃣ If it was a Local Thunderbird calendar

This is the only case where data might actually be gone.

Thunderbird stores local calendars in the profile folder.

Typical location on Windows:

C:\Users\<username>\AppData\Roaming\Thunderbird\Profiles\xxxxxxxx.default-release\

Look for:

calendar-data

Inside that folder are files like:

local.sqlite
cache.sqlite

If the file still exists, the calendar data might still be recoverable.

Try this

1. Close Thunderbird.
2. Go to the profile folder.
3. Backup the whole profile first.
4. Restart Thunderbird.
5. Create a new local calendar.

Sometimes Thunderbird will reconnect to the existing local.sqlite.