outlook.PST file (often happens if message store is changed).
Search your hard drive(s) for all files named OUTLOOK.PST. Each that you find can be opened in outlook by selecting to do so from the Files menu.
If you do only show one and it is large and you absolutely see no messages in any folder under Personal Folders take a look at the View Menu (must be in Inbox or another folder) then Current View and make sure that either Messages or Messages with Auto Preview is selected rather than Unread or Last 7 days (both of the latter will hide messages from view).
If that is correct then you might try clicking once on Personal Folders then Properties then the Advanced button and Compact Now.