Update it then. Right-click…

Did the updated Thunderbird not offer to check for necessary updates on first run?

Thanks for your response. I tried to update the add on but I just get the same message about Lightening not being compatible. How can I go back to a previous version?

And no, it did not offer to update, I am on the release update channel.

This is seriously unreliable!

In order for us to better investigate this issue could you please include the troubleshooting information to your post To find the Troubleshooting information:

• Open Help (or click on three-line-icon and select Help)
• Choose Troubleshooting Information
• Use the button Copy to clipboard to select all. Do not check box "Include account names"!
• Paste this in your post.

Did this occur immediately on update to 38.6? Did it occur on previous version of Thunderbird? What was the cause of the previous calendar going missing this week?

Issues of this nature are affecting a very small number of users, but to fix it we really need to establish the exact cause. Hence all my questions.

Hang on that is Thunderbird 38.5.1.... a new update would be 38.6

How did you fix your last issue? It looks to me like your fix might be the cause of this round of issues.

Thanks for spotting that. For some reason it had reverted to the previous issue. I was following instructions from Christ1 on a previous calendar query. I have just re-installed38.6 and my calendar has returned. Thanks for your time.