Replication lets you enable warnings for several conditions. This includes imminent subscription expiration. Subscriptions can expire if they are not synchronized within a specified retention period. For more information, see Subscription Expiration and Deactivation.
When you enable a warning by using Replication Monitor or sp_replmonitorchangepublicationthreshold, you specify a threshold that determines when a warning is triggered. When that threshold is met or exceeded, a warning is displayed in Replication Monitor, and an event is written to the Windows event log. Reaching a threshold can also trigger a SQL Server Agent alert. For more information, see Setting Thresholds and Warnings in Replication Monitor and How to: Programmatically Monitor Replication (Replication Transact-SQL Programming).
The resolution for this issue depends on the reason the warning was raised:
If the threshold has been exceeded, but the subscription has not yet expired, synchronize the subscription. For more information, see Synchronizing Data.
If the agent has been running, but has not been replicating changes properly, this can cause the subscription to expire. For transactional replication, make sure that the Distribution Agent and Log Reader Agent are running. For merge replication, make sure the Merge Agent is running. For information about how to start these agents, see How to: Start and Stop a Replication Agent (SQL Server Management Studio) and Replication Agent Executables Concepts.
If the subscription has expired, it must either be reinitialized or dropped and re-created, depending on the type of subscription and how long it has been expired. For more information, see Subscription Expiration and Deactivation.