Service for a customer's subscription has the Stopped status. On the 'Service Status History' tab in the subscription properties the following entry is present:
Service Stopping: Automatical Synchronization: Stop Service Triggered by daily billing process
Moreover, in some cases there is a renewal order cancelled by Billing. Why BA stopped the subscription service?
Every day the daily billing process check the expiration date of the subscription and stops the service of expired subscriptions.
If the is an unprocessed renewal order (in the status "Waiting for Payment" or "Service Not in Sync" or some other status different from "Completed"), this order gets cancelled.
BA stops the subscription service after subscription period and grace period passed, so check Service Terms which are assigned to the subscription. If Grace Period is 0 days, subscription will be stopped right after expiration, otherwise it will be stopped after grace period is over.
The customer has to renew the subscription to reactivate its service.
You may find more information about Service Terms in the BA guide..