Click “Manage Navigation” from the Home page.
Select the broken sub-menu group and view its contents. Please take a photo/screenshot of the current contents of the sub-menu, you'll need this information later.
Once selected, delete the sub-menu group by clicking "Remove Selected Item".
Locate the sub-menu group you've just removed and click the edit icon.
Populate the "Link Label", "Start Publishing", and "Stop Publishing" fields and click "Add Sub-Menu Group".
If your navgation tab has moved, please click "Left" to return to your navigation.
Drag & drop the sub-menu group into the navigation.
Using the photo you took earlier as a guide, click & drag the webpages back into your sub-menu group.
In the upper right corner of the page, click "File" and then "Save Menu".