csa closed case £11.000 arreas still owed

HI,single parent mother of two children.
i phoned csa today to discus arrears they then told me case has been closed (news to me )since i opened a case start of 2012 which was set up to make sure i received csa ongoing payments from childrens farther which was nothing to do with arrears payments as i thought they were still dealing with. can anyone enlighten me what i need to do to receive arrears?? as csa are not being helpfull.:(