Do they appear after Help > Troubleshoot mode? If not, does repair folder help, from right+click on folder > properties?

I had a problem with missing emails with version 115.10.1. Thunderbird updates automatically, so I'm not sure when that occurred. My missing emails were all "read" emails and if I read a new email, it would disappeared when I closed it after reading it. Is that what's going on with you, too? If so, we figured out how to make the "read" emails reappear: Right above the message pane, is the search box for that mailbox and to the right of the search box are five icons. These are toggle switches. The first icon is "Show only unread messages". If only your unread messages have disappeared and you click on that icon, I think your missing messages will reappear. I'd never noticed those five icons before. I thought we must have inadvertently clicked the icon, but If you're having the same problem, I think TB v 115.10.1 must be responsible.