Unfortunately skype has horrible to non existent customer service. They make their money by providing a cheap or free service and spending as little as possible supporting it.
Try these instructions to get it working though.
login to your account at www.skype.com
scroll all the way down to the little link that says STORED PAYMENT DETAILS
if it shows the credit card there see if there is a way to delete it
then click add new payment method
add the card back
then click on billing information
make sure that your address there matches EXACTLY the address where the credit card bills are sent.
If that does not work than a work around is to create a paypal account at
www.paypal.com and put the credit card in there
then you can use the paypal account to pay skype by selecting paypal as the method for payment. and paypal will deduct the amount from the card.