In most cases you should have a credit card on file for renewals.  However, it could be that your credit card has expired or is not up to date.  To ensure your domains will be renewed:

1. Login to your account at https://secure.registerapi.com/resell.php?goto=account&siteid=2...
2. Click Billing & Order History
3. Click Assign Payment Methods to Services
4. Select Use a new Credit Card in the drop-down next to the selected service
5. Click Use this payment method
6. Enter your new credit card information 
7. Click Update

This credit card will be assigned to your domain(s) auto renewal. To assign this credit card to all services, select this credit card for the Global Payment Assignment.

You should also check to make sure your domain is set to renew automatically.  Click on your domain name and make sure it says "Auto-Renew: YES".