It is also saying the my calendar is momentarily unavailable and theres a yellow triangle with a ! Inside of it...

any update on this !! I have the same issue on my mac. I've only joined the support forum today but have noticed there are nowhere near as many replies as I'd expected or hoped for on a lot of questions posted here. I rely heavily on Thunderbird and the fact it has wiped the calender is very disturbing and a knock to my confidence.

We solved it. You know on the left hand its written calendar, and then under its written the name of the person/calendar. Well it was saying that, that specific calendar was momentarily unavailable. we double clicked it and it gave us the option of activating the calendar so we checked marked it. When we clicked ok or save it asked us to connect to our google account. After doing so we checked our calendar on google and found all of our events. Turns out we were lucky that thunderbird was saving the date we entered in gmail as well. We were never able to retrieve them on thunderbird, we had to retype everything. Sorry i am recounting this from memory so i might not be describing this 100% accuratly. I hope this helps you!

nope this didn't change a thing sorry, I deactivated the account then reactivated it, I added a new event on my iphone about an hour ago and it popped up straight away on my thunderbird calendar but still nothing I have written previously.