How to drop all database connections?

Oct 23 2009 9:02 AM

Hi, I'm trying to drop the connections of a database using this command :
"alter database dbName set single_user with rollback immediate "
But u got this error "ALTER DATABASE failed because a lock could not be placed on database 'dbName'. Try again later."
Please I need help as soon as possible.
Regards
Tamer Khalil