This can happen when Outlook doesn't properly recognize the PST file being used to store data.

Click on Tools > Account Settings. Then click on the Data Files tab. If the box there shows up blank, then you'll need to add a new data file. Sometimes just the process of adding a data file and then closing and re-opening Outlook will be enough to cause the original data file to show up. If that doesn't work, then you'll have to actually import your data from the old file to the new one.

Hope this helps.

-SW