Generally this happens if we were unable to process payment on the last scheduled monthly billing date. This is likely due to an expired credit card, changed billing information, or lack of funds in your account (if you're paying via PayPal).
We will automatically attempt to bill you again in five days time. If payment goes through on the second attempt, your subscription will be reinstated automatically.
So please check and fix any issues with your payment sources, then wait five days for the next automated billing attempt. For help updating your billing information, please see this page.