why is my account not allowing me to purchase any songs

bought a few songs unknowingly using my pre-paid visa and used up what was left of the money in it and I had already bought an iTunes card so I tried to buy more songs with it but when I try to buy anything I keep being told that there's something wrong with my billing information and won't allow me to purchase anything else until I fix it. I then go to "fix it" by deleting my card information and it won't let me. there's no more money on the card and my iTunes card is in my iTunes account but it still won't let me buy anything. WHAT DO I DO?!