You do not mention what version of Outlook you use.
Without knowing this I will include the instructions for
Outlook 2003:
Open Outlook
- Select the VIEW menu
- Select ARRANGE BY
- Select CURRENT VIEW
- Select DEFINE VIEWS
In the Custom View Organizer window:
- Select DAY/WEEK/MONTH
- Click the RESET button
- Click OK when the window asks "Are you sure you
want to reset the view "Messages" to its original settings?"
- Click the APPLY VIEW button
- Click the CLOSE button
This should reset your Calendar to the original settings.