When one of your admin pages goes missing, you can usually recreate it.
1. Log into another portal on the same installation or another DNN installation with the same version number
2. Go to the page that is missing in the original portal
3. Make a page template of that page
4. Copy that template to your original, broken portal
5. Make a new page, name it the same as the missing page, place it under the correct parent page
6. Apply the template to the page, overwriting the original
That should do it. You should now have a working replacement page for the one that went missing.