Originally Posted by Blizzard
If your subscription ends and there is no payment information listed on the account then it will become frozen and inaccessible for play. Provided that the characters do not get deleted by the account holder, we will retain all character information on our servers indefinitely.
You will still be able to access your account through the Account Management page. From there you can set up a new subscription for your account at any time, which will allow you to log into the game again and pick up right where you left off. More information on getting the account started again can be found here.
If you have any questions or concerns about your account or subscription, please Contact Billing and Account Services for additional assistance.