Currently we are working on new interface for payment gateway which will give you all the information about renewal and it will be available from our next release. Usually we follow auto renew process once a year for the credit you've purchased, since your billing info has been failed you need to follow the link for renewal: URL:https://support.syscloud.com/buy.aspx and Select option #1: shown in the snap (attached), Enter required number of additional users. This will calculate a total payment of $XX (Users/Year). You will be mapped to the plan 'SysCloud Backup' Renewal Price at $X/User/Month with Unlimited Storage.
You can start backup for any unprotected users in start backup page and make use of the remaining credit.