Based on the description, your messages probably have been archived automatically.
Please verify by the following steps:
1.Click the File tab, and then click the Options tab on the File menu.
2.Click the Advanced tab.
3.Click AutoArchive Settings .
4.If the checkbox Run AutoArchive every is checked, your old messages may have been archived to a .pst file.
If you have archived your messages, you will see Archives on the Navigation Pane. You can view your archived emails in the sub folders.
To know more information, please visit Description of the AutoArchive feature in Outlook:
http://support.microsoft.com/kb/830119/en-us
If your issue persists, please feel free to contact me.