Step 1: Download and extract the MFCMAPI tool from http://mfcmapi.codeplex.com.
Step 2: Launch mfcmapi.exe and click OK through the intro screen.
Step 3: On the Session menu, click Logon.
Step 4: Select the Outlook profile for the affected mailbox, and click OK.
Step 5: Double-click the email address and navigate to the missing folder.
Step 6: Right-click the PR_ATTR_HIDDEN property and select Edit Property.
Step 7: Uncheck the Boolean checkbox and click OK.