Hello, I’m Nathan, and I’ll be assisting you today.
Sorry to hear that you’re having trouble with this but I’ll do my best to help you out.
If needed, you can use this link to reset your password:
Then, from your amazon account page on the computer,
First, go to Manage Payment Options:
there you can be sure your card/bank info is correct (right address, valid expiration date, etc)
From there, go to Manage Your Content and Devices:
On this page, click on the SETTINGS tab at the top right,
then you can from there set a default card to be used for one click purchases on the kindle.
Under Kindle Payment Settings, click on the Edit Payment Method button, and you can set the card you want used for your kindle purchases.
After that, reboot the kindle by pressing/sliding and holding the power button for 20+ seconds, and it should operate normally from there.