Users of my website and email cannot access their accounts. I logged into the management site and noticed that our primary domain is missing  in he Add/Manage Domains section. When I type the domain name that you want to redirect I get this error: Domain Name Already Registered With Us.

How do I fix this????!!!!

Hi! 

As you have mentioned that "Domain Already Registered with Us" then, it means that the domain name is already registered with Microsoft Office Live. However, it could be if you have by mistakenly deleted the domain from your Microsoft Office Live account. Hence, in order to confirm, you should contact the Support Team of Microsoft Office Live.