What is wrong with the calendar? In what way is it frozen?

I had power failure while on the computer using Mozilla Thunderbird, the computer automatically restarted and that's when the calendar froze and I lost all data and now it won't work although I can still received and send emails. I closed the computer and rebooted but no change

I’m (still) not sure what you mean by "frozen", but if you simply see no events, are you able to create new events or tasks that do display? And did you make sure the calendar(s) is/are still ticked in the left pane?

All the events previously posted on the Calendar have disappeared and I am unable to create new events or tasks and yes the calendar is ticked in the left pane. When I open Mozilla Email on the top left hand corner of the computer an error message appears and on the screen it shows moz-storage-calendar://

It looks like Thunderbird can no lonner find your internal calendar file, represented as moz-storage-calendar:// in the calendar’s properties.

Try this:

1. In Thunderbird, open your profile folder by going to Help > Troubleshooting Information and clicking Open Folder, next to Profile Folder.
2. Close Thunderbird.
3. In the file explorer with the profile opened, go to the calendar-data subfolder and drag the local.sqlite file to this web page. This is you calendar data - creating a copy of the file locally would be a good idea.
4. Select cal_events in the drop-down at the top of the web page.
5. In the profile folder opened, copy the prefs.js file to a temporary location on your computer, and create another copy of it there.
6. Open one copy of the prefs.js file and look for the entry starting with calendar.registry.xxxxx... and ending with moz-storage-calendar://, for example (the number is random and can vary)
   user_pref("calendar.registry.3d082366-4923-91da-23ae-acf85c295bc4.uri", "moz-storage-calendar://");
7. In the editor, compare the numbered (and probably identical) cal_id column values on the web page on the left side to the numbered part of the string above in the prefs.js copy.

Do the entries match? Note that there could be 2 or more if you you have 2 or more calendars, and each of them should have a matching line ending with .uri above, as well as a set of other parameters.

If not:

1. Edit the copy of the prefs.js file: replace the numbers found in all calendar.registry.xxxxx... entries with the values found on the page, especially the string above ending with .uri, since this identifies your calendar location. You can hover the left colum to see the full numbers, or click and copy them entirely.
2. Save the edited copy of prefs.js, place it in the profile folder (replacing the existing file - make sure you have it backed up) and start Thunderbird.

Does it work?

If the entire user_pref("calendar.registry.3d082366-4923-91da-23ae-acf85c295bc4.uri", "moz-storage-calendar://"); line is missing in prefs.js or there are no lines starting with user_pref("calendar.registry at all, create a new calendar in Thunderbird as a first step and redo the steps above. The new calendar should create a set of (at least 4 or 5) lines in prefs.js. Then edit all of them to include the numbered part found on the web page.

See this question for a little background.

Tonnes thanks so far it has been very helpful but I am stuck at point 6 as the file when copied and placed in another area on the computer won't allow me to open it, so I am stuck at the moment

Open one copy of the prefs.js file and look for the entry starting with calendar.registry.xxxxx... and ending with moz-storage-calendar://, for example (the number is random and can vary) user_pref("calendar.registry.3d082366-4923-91da-23ae-acf85c295bc4.uri", "moz-storage-calendar://")

This could happen because the editor does not know how to open .js files, or perhaps you double-clicked it (which would treat it as opening a JavaScript file)?

If you use Wordpad, use File (arrow) > Open and select All files (*.*), or temporarily rename the file to contain the .txt extension. I prefer the first option however, as renaming it could change the format when saving.

Tonnes I can see all the missing data from the calendar I have copied the following from the word pad - user_pref("calendar.registry.7ad3aeef-e5af-423a-8ee6-a0aa81be8a1e.uri", "moz-storage-calendar://") but it won't paste into the copy of 'prefs' that I saved in another part of the computer so I am stuck at the moment

You should copy the string found on the web page (i.e. from your actual calendar file items) to a copy of the prefs.js file on all spots for the calendar’s "registry" entries. If you managed to copy that prefs.js file elsewhere and open it for editing, you should be able to save it after pasting the string on those lines and save the file, provided that you didn’t save that copy to a system location (such as c:\windows) but e.g to d:\ and that the file is not write-protected, which it should not.

Example: - Calendar strings (on web page) for example show 1ad1aafe-5aaf-a43b-ee6b-a0aa81be8a1e7 (1) - prefs.js contains user_pref("calendar.registry.7ad3aeef-e5af-423a-8ee6-a0aa81be8a1e.uri", "moz-storage-calendar://") (2) - Replace the bold strings in 2 (and the matching entries with the same numbers) by the one in 1 so that all entries show user_pref("calendar.registry.1ad1aafe-5aaf-a43b-ee6b-a0aa81be8a1e7.uri", "moz-storage-calendar://") - Save the prefs.js file. - Copy the saved prefs.js file back to the profile folder when Thunderbird is closed.

Tonnes Sorry things not working

I’ll send you a PM to see if this can be solved another way.