Your subscription may have lapsed. If you are currently subscribed, your should open Castle and tap Settings, then tap Subscribe to Premium. Then tap Restore subscription.

If your subscription has expired, you must renew your subscription to keep using Castle Premium. To do so, open Castle, tap on Settings, then tap on Subscribe to Premium. Choose your term and proceed with your purchase. Your new term will start immediately.

You can check your expiration date by following the steps under How do I stop or change my subscription.