Try this and see if it fixes ur problem.
select * from master.sys.sysprocesses
where spid>50 -- don't want system sessions
and dbid = DB_ID(name of database in question)
--Find the SPID which is making connection to your single_user mode DB and KIll it
Kill 100 -- replace 100 with the SPID which you get from the above query
ALTER database 'your DB name' set MULTI_USER with ROLLBACK IMMEDIATE