Version of outlook? Did the folders disappear or the entire section?
In both OL 2003 and 2007, press Ctrl+1 to make sure you are in the mail
view.
In 2007, the folder section can expand and collapse - there is a small pair
of carets on the left side to toggle the open/closed state.
** Please include your Outlook version, Account type, and Windows Version
when requesting assistance **