Outlook.pst not a personal folders file

I run Outlook 2000 on a home pc (Windows XP). Everything was fine until today, when I was inexplicably no longer able to access my personal folders. When I click on “Outlook Today – [Personal Folders]”, the following message appears on the screen: “Unable to display the folder. Microsoft Outlook could not access the specified folder location. The file C:\Documents and Settings\Joshua Fireman\Local Settings\Application Data\Microsoft\Outlook\Outlook.pst is not a personal folders file.”

How do I re-designate the file as a personal folders file?