There are a couple of reasons you could be unable to purchase postage USPS due to a credit card issue.
If you have not received a new credit card:
It may be best to contact your bank to ensure that they will allow the charges to go through. As postage is considered a form of currency, there are strict regulations around postage purchases so the banks can be quite strict.
If you have received a new credit card:
You will need to update the information with your USPS postage account. Find out how to update your billing information.