MS-SQL: Changing single user Database to multi users
Changes to the state or options of database "DatabaseName" cannot be made at this time. The database is in single-user mode, and a user is currently connected to it.
alter database "DatabaseName" set multi_user;
If there's users connected to the Database and you don't know who, you can use "sp_who2" to check who's connected.
You can kill only user process by there SPID: