Just a thought, if you using PST has your file size gone over 2gb as Outlook has a problem with sizes over the 2gb limitation, it just ignores the additions beyond that, opening a new file at around 1.8gb will avoid this problem.
Also check were the auto archive folder is as old messages can often be auto transfered into this location,
To check whether Auto Archive is enabled, click on Tools, Options, then the "Other" tab. One of the buttons is labelled "Auto Archive". Click on it. If the option at the top is selected then the auto archive feature is enabled. By default it archives all email that is older than six months automatically.
However, before you disable the feature, you need to make sure this is what has happened. About half way down you will see "Move old items to" and a path name. Close the options dialogue and then click on File, Open, and Choose "Outlook Data File" or "Personal Folders" (label depends on the version of Outlook you are running). Then browse to the path that you have just seen above. When you open the file, you should see your old email. If so, you will need to copy it back. Old Calendar and other items will also be stored in there.