If you try to access the database which is already in the Single-User mode, you need to close all the connections to the database first, otherwise you will get an error message:
Msg 5064, Level 16, State 1, Line 1
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.
Msg 5069, Level 16, State 1, Line 1
ALTER DATABASE statement failed.
The following query kills the processes accessing the database:
-- Create the sql to kill the active database connections
declare @execSql varchar(1000), @databaseName varchar(100)
-- Set the database name for which to kill the connections
set @databaseName = 'DatabaseName'
set @execSql = ''
select @execSql = @execSql + 'kill ' + convert(char(10), spid) + ' '
where db_name(dbid) = @databaseName
DBID <> 0
spid <> @@spid
Then you should be able to bring the database back to Multi-User mode as usual:
ALTER DATABASE 'DatabaseName' SET MULTI_USER