I can remember my 3 digit code as it was a while ago and I got rid of the prepaid card and now I can't redem my amount how do I get it back? Anyone please help

My old computer crashed and I have recently purchased a new one and downloaded Itunes on it but when I go to purchase music on my account it ask me to verfy my account..I used a prepaid visa or Itunes card and can not remember the 3 digit code as I discarded the cards when I inputed the information. I have a balance on my account but can't use it until I put in this code...is there another way to get this balance or is the money on my account no good now....can anyone help me please. Its not alot of money $17. something but why should I loss this amount and they benifit from it.