payment details are being declined

I have recently changed the address of my credit card payment details so have been requested to re-verify via iTunes. after editing my information i get a message saying my card is declined and i should input a different card, (which i don't have).....id like to ask if anyone has and advise that could assist me as i cannot update, download from app store or make purchases.

I have checked my details when entering them and i know these are correct, logged into iCloud on my iMac and repeated the same and still same thing.

I have an iphone5 with iOS 9.3.3

Thanks