Putting Databases In Single User Mode
You can run sp_who2 or equivalent to find a connection that's using that database. Assuming you're a sysadmin on the box (and, if you restarted the SQL Server instance, I hope you are), then you should be able to kill the process that is the current single user.

I would enter the kill command in a query window, along with the command to restore multi-user mode:

KILL 123;
ALTER DATABASE &lt;your_db_name&gt; SET MULTI_USER;


It is still possible that someone will manage to grab the connection in between the execution of the two commands (or that the current connection will need to roll back some changes); repeat as necessary.

NOTE: You are risking wiping out someone's changes to the DB in question by killing their connection. Please pay attention to the user in question, how they're connected, and whether it seems safe to kill their connection; you may want to contact them before killing their process.