If your calendar was synced with an online service (like Google Calendar), re-add that account in Thunderbird to restore your events. Or contact Thunderbird support page for assistance.

Thanks. No it wasn't. I just assumed it was within the profile.

How do I contact Thunderbird Support? All I can find are QAs which don't answer my problem and link to this forum

This forum is Thunderbird support. I don't know if someone has transferred the topic from Firefox. They are pretty good at simply changing a topic that has lingered for a time and had responses without any sort of comment as to what they are doing.

To answer your question, yes your profile contains your local calendars. But how did you get you profile into Thunderbird. I have seen folks changing the account directory and thinking that is changing the profile folder location. It is not.

If that is your process, your profile was probably still on the drive you formatted.

Hi Matt. Thanks, I did actually post this in the Thunderbird forum.

After installing Thunderbird I used the profile manager, created profile and chose the folder where my profile was located

Hi Matt

I have just gone to start adding in dates and they're all there again! Very strange as they were definitely not showing yesterday.