If you can see the database while logged in as you, but not as SA then it's a permissions issue. The SA must have been restricted from that database. Check with the database administrator. If you're the database administrator, log in as yourself and check SA's permissions.
If you can't see it no matter who is logged in, it could be that it's detached or offline.
Also, this would be better placed on ServerFault.com.
Was this database recently restored from a different server? I can't believe i didn't think of this because it happened to me just last month... If this was restored from a backup made on another server, then I have another possible option...
You and I see the login SA as "SA" but under the hood, SQL Server sees it with a different ID. (I think it's a guid).
From time to time, I have to take a backup of DB from our live server and restore it onto our test server. When that happens, the permissions are not set correctly for any SQL Server logins (but they are fine for Windows logins.) I can look at a table, for instance, that shows that SA has access, but when I try to connect using SA, I got the same message.
What we finally figured out was that we needed to DELETE the SA user from the database, then DELETE it at the server level, then set it back up again with the proper permissions.
This is because of the discrepancy between the Id's that SQL Server uses under the hood for the "friendly" username.
For example, on our test server, SA may have an id of abc and on our live server, the id may be def. Somehow this messes things up when restoring a DB from one server to another.
There's an article about it here: http://searchsqlserver.techtarget.com/tip/Restoring-a-database-from-another-SQL-Server
Oops. You have to register for the one above. Here's one you don't need to register for.
That last one contains links to a KB that shows a better way to fix this than the way I'd been doing it. That figures. So even if this doesn't answer your problem, you've helped me find a better answer for mine, so +1 to you for asking the question and helping me out.