This may help you to get rid of your problem:
1. Right click on the .pst file name in your folder list.
2. Choose Properties (Data Files Properties in Outlook 2010/2013), then Advanced button.
3. Click Change Password button.
4. Type your password in the Old password field.
5. Leave the New password and Verify fields blank.
6. Click Ok and work your way back to Outlook.
This dialog is also accessible from File, Account Settings, Data File tab. Select the pst file and click Settings.