I read several articles in the forum, and attempted the suggested fixes to no avail, so a few facts first: Thunderbird version 45.3.0 Lightning version 4.7 Windows 10, version 1511 When I started Thunderbird this morning, I got a popup that told about Lightning functionality being included in all versions of Thunderbird from 38 on, and was given a choice of disabling the functionality or not. As I've been happy with Lightning I agreed to continuing with the calendar functionality enabled. An immediate problem was that Thunderbird did NOT start with my existing profile! Then I noticed that my calendar had no entries. As both seemed to involve my profile, I manually configured Thunderbird to start with my existing profile, but the results were mixed: ALL messages still on the message server I use were downloaded, suggesting that Thunderbird had lost it's recent incoming message history. As the message server only stores only 91 days of messages, I had lost everything prior to June 28. I had a secondary copy of the profile, but as it is a continuous backup, when Thunderbird overwrote the incoming messages the backup copy was marked as deleted, so I may not be able to recover anything (boy is that a hard lesson to learn!) The local.sqlite data appears to be intact in the calendar-data folder, but not in the calendar-data.sdb folder. Then after following the recommendation to remove Lightning in safe mode, then restart to allow Thunderbird to rebuild its enviroment, I still had no calendar. Like the person who commented that the necessity to manually re-install Lightning wasn't clear in the support article, I found the reply about reloading Lightning. I still do not have any calendar functions. Going to go have some lunch to give the volunteers a bit of time to think about what might help me, but I've got to have mail and calendar functionality working, so if I get no suggestions, I'll save off copies of all the profile data, then uninstall and reinstall Thunderbird from scratch.