Won't display any read / saved emails

Using Thunderbird 128.8.1 on Windows 10 (yes, I know). Have used Thunderbird for many years, with thousands of saved messages in local folders. When I started the program this morning, no previously "Read" messages of any kind (local or online) appeared in any folder -- just the new, Unread messages in the Inbox. If I read a new message and then go on to another one, the first message also disappears, whether I leave it in the Inbox or move it to another folder. In Troubleshooting Mode, all the "Read" messages show up, so they're clearly still accessible. I feel like I must have accidentally toggled a switch not to display "Read" messages -- but not sure how to reverse the process. Thanks for any help!