I purchased Starz through Apple TV. We have been able to access it one time, when we bought it. I get billed for it , but can not view anything on Starz

Is this happening on the Apple TV box, or in the Apple TV app on a third party device? Which model?

Did you subscribe in the Apple TV app, as an in-app Channel, or did you start the subscription in a separate Starz app? These two types of subscriptions are (generally) not exchangeable. You have to watch it in the app where you subscribed.