I never had this problem and only been experiencing it recently. After turning my computer off, turning back on and loading up Thunerbird, my calendars are not displaying again. I have to keep removing and readding again. I am using Google Calendar that is linked to my phone and pc. The phone is fine and always displayings my appointments. I am using the "Provider for Google Calendar" extension on Thunderbird. I at first though it could be OneDrive interfering with the data, but that does not make much sense as its a calendar hosted on Google, but still I configured and checked the OneDrive. I am at a loss and getting fed up of having to readd my calendars back every day. So, I don't know if this was an update that did it, a broken extension, etc. But, I am desperate for a solution. Regards