the cause is moving files that are used in the program.
moving these file and not telling outlook that you moved them can either cause outlook to re-create, fail to start, or just sit there doing nothing.
one way to get these files back is to remove and re-add the software.
another is to click file -> account settings -> select the data files tab.
and select a new location for the data file, or select the data file that you have moved.
the correct way to move a PST file is to right click and close it. move it then select to open it again.
the correct way to move an OST file is described here: