Emails disappeared - folders empty

I am using Thunderbird 115.5.2 (64-bit) on an older iMac on OS 10.12. I have four email address accounts, all gmail, and using POP. I have stored emails I wanted to keep in sub folders in Local Folders. This has all worked well for years.

Yesterday I lost lost all content from some of the subfolders in Local Folders, while some retained some content and a few seem intact. I also lost most emails that had arrived in the inboxes in the last week. The folder pane still shows the accounts, with their own set of folders. As usual, the inbox folders are highlighted with the number of new emails downloaded from the Gmail server to each address, but I can not see them in the message pane, or find them anywhere else. I sent a test message from one of my addresses to another – It didn't show as sent in Thunderbird, but did go through the gmail server and the message count of downloaded messages on the recipient address went up by one, but I couldn't see the email. Then I tried clicking on the folder "properties" then 'Repair Folder' but nothing happened, although the information on that window showed the number of messages and MB that I would expect to be in that folder, rather than what was there, which presumably means they may still be in there somewhere? I would be very grateful for help – I'm not tech savvy and feeling completely lost.