As a prepaid service, each account is only permitted a certain amount of overage before a mandatory payment must be made.  If you receive an email that you are approaching your credit limit, or that you have exceeded your threshold, you should immediately login to your account and make a one-time payment to ensure that your services are not temporarily suspended.  You can do this by logging into your dashboard and going to the SETTINGS --> ACCOUNT page, and process a one-time payment.