The article [Q295965] states that when OE is started, some existing folders (Inbox, Sent, and so on) or personal subfolders may be missing and can be caused if the file "Folders.dbx" (the Indexer) is damaged or corrupted. To resolve this issue, rename or simply delete this file for the impacted identity, and then start OE which rebuilds it. Additional reading concerning the "Folders.dbx" file, [Q233264].
CAVEAT: The Folders.dbx file keeps track of Active News Group Subscriptions and the information is lost when the file is deleted. Again be advised, the file Folders.dbx is the file which categorizes and maintains the default mail Store (in OE, click Options, the Maintenance tab, click Store Folder, and then make a note of the path) and default OE files. If a certain file or folder is moved or copied into the mail Store in lieu of using the correct Import method, the Folders.dbx would not be aware of this change and therefore would not list it. When copying or moving files into a mail Store, delete the Folders.dbx file before OE is started.