Have you hidden the Folder Pane, under View/Layout, or are Local Folders visible, but not the created subfolders? Post a screenshot.

sfhowes response is somewhat unclear. There does not seem to be any View/Layout options. My folder pane contains the following folders >rleerobison

  inbox 

>Trash >Local folders

   Junk
   Tash
   Outbox
  Drafts-rleerobison
  sent-rleerobison

sent-rleerobison is not the sent folder where my sent items go. That, too, is missing. It was not missing when I sent my earlier message. In fact, I sent a very critical email and checked to make sure it went to the Sent folder. It was there. Now the folder is gone.

All the folders that are missing, including the "Sent" folder, were subordinate to Inbox.

I presume this is a POP account, so open the profile folder from Help/More Troubleshooting, Profile Folder, and open the Mail subfolder. Inside, there should be a subfolder named after the server, e.g. Mail/pop.gmail.com. In that folder, see if there are mbox files - files with no extension - named after the missing folders, inside Inbox.sbd. If the mbox files are there, close TB and delete or rename the folderTree.json file in the profile folder, then restart TB and see if the folders reappear under Inbox.

Don't know if I have a POP account, but I followed the instruction and did find a "mail" folder. Under it is only one sub folder called "Local Folders."

"Local Folders" contains drafts-rleerobison drafts-rleerobison.msf filertlog junk junk.msf msgFilterRules.dat sent-rleerobison sent-rleerobison.msf trash trash.msf unsent messages unsent messages.ms.

There isn't any subfolder named "Mail/pop.gmail.com" or anything like it. btw, My primary account is not gmail, but icloud.com. If that helps?

Also, there is a folder called "ImapMail" but when I open it, I am obviously out of your described folder tree and on unfamiliar ground.

The Server Type is shown under Server Settings in Tools/Account Settings.

It looks like the account is IMAP, so right-click the account in the Folder Pane, Subscribe, Refresh, and see if the missing folders are listed and can be subscribed.

We are on the right track. I see the folders when I click on subscribe. But "Refresh" does not seem to do anything. I checked the boxes and clicked "Refresh" still nothing.

Amend that last! I closed then reopened the account. But the folders do not seem to be up to date (nothing after Jan 1, 2021) and I do not see the "sent" folder. So there is still a problem.

Further Amend! SOLVED (I think) I just sent another email and the "sent" folder appeared with (apparently) all sent items from the past. Also, it seems all other folders are up to date as well So Solved Thanks.