Some one is connected that db...

Do you any monitoring tools connecting to this db...

What is the error you are getting...when you try change it to multi user mode?

Try the following script...

DECLARE @ID INT

DECLARE @MSG CHAR(8), @DataName sysname

While 1 = 1

BEGIN

    Set RowCount 1

 SELECT

  @id = spid

 FROM

  Master..Sysprocesses P,

  Master..Sysdatabases D

 WHERE

  <A href="mailto.Name=@DataName">D.Name=@DataName 

 AND

  D.dbid = P.dbid

    IF @@rowcount = 0

    break

    SELECT @msg = 'KILL ' + convert(char(8) ,@ID) 

    Print @msg

    EXEC( @msg)

    --break

END

GO

ALTER DATABASE....

GO