perhaps post an image of what you see.

Hi Matt ...thank you for responding. I do not know how to post the page but I will try to describe it below It is a page with "Thunderbird Mail" and my email address on it in heavy print ( this is under the header with "get mail , write, chat, Address book, tag, quick filter and search box" On the left is a margin the width of the usual folders but it is empty & pale blue Under the heading ( to the right of the margin ) are three boxes with headings

Email  (sub heading: read messages & write a new message ..active)

Accounts(sub headings: View settings for this account ..active & Create a new account ..not active)

Advanced features with various headings starting with "Search messages" If I click on this one and select one of my em addresses it reveals a menu with all the folders & sub folders So all the TB seems to be on my computer but I can not access it by clicking on the icon as I used to be able to get the start or home page I would be very pleased if you could solve my problem Thanking you in anticipation …. John Z

There is a number of reports of something that sounds like what you are reporting. The instructions are not specific to this, but they will get the job done as the same file needs to be deleted.

https://support.mozilla.org/en-US/kb/rebuilding-global-database