I somehow have lost my SENT, DELETED, and THRASH files in my inboxes. I know they still exist if I undo a DELETE command, but can't find them in my list, HELP!

I did not change anything except update Thunderbird according to notice I received. After the update was completed, my SENT, DELETED, and THRASH tabs I used to find under INBOX had disappeared. That happened for both of the INBOXES I had to store my RECEIVED or SENT emails. Anyone ever experienced this disappearance? I appreciate any help I can get. I often go back to the sent file to store emails I have sent. I also have occasion to recover a THRASHED email that was discarded by error. Now I can't do that, since I cannot find the THRASH or DELETED file.