Welcome, I am Cory. Thank you for asking your question today.
Please login to the account at http://www.amazon.com
Once you login, please click https://www.amazon.com/mn/dcw/myx.html/ref=myk#/home/settings/payment to manage the settings for Kindle payments.
Edit the payments.
Delete the old one completely and add the new one.
You should be able to buy books again.
Let me know if it is fixed.