Some availability replicas are disconnected
Availability Replicas Connection State
Some availability replicas are disconnected.
This policy rolls up the connection state of all availability replicas and checks for any availability replicas that are DISCONENCTED. The policy is in an unhealthy state when any availability replica is DISCONNECTED. The policy is otherwise in a healthy state.
For this release of SQL Server 2016, information about possible causes and solutions is located at Some availability replicas are disconnected on the TechNet Wiki.
In this availability group, at least one secondary replica is not connected to the primary replica. The connected state is DISCONNECTED.
Use the availability replica policy state to find the availability replica that is DISCONNECTED, and then resolve the issue at the availability replica.