the box that contains the inbox/trash/drafts/archives, etc. has disappeared. I don't really unders

the box that contains the inbox/trash/drafts/archives, etc. has disappeared.

I don't really understand what that means. What box are you talking about?
There is a main Thunderbird window (as opposed to a Write window) with a folder pane, and the folder pane has any of inbox/trash/drafts/archives, etc.