If it happened on the Primary DB server and DB cannot be accessed. Restart secondary first then primary. See if the Not synchronizing changed to Not synchronizing / in recovery. After there, its a waiting process.
It may change to revert / in recovery or synchronized depending on DB and DB log size.
Checking synchronization state
SELECT @@servername as SQLName,db_name(database_id) as DBName,synchronization_state_desc,database_state_desc
from sys.dm_hadr_database_replica_states where is_local=1
Checking performance counter for redone, undone, recovering queue.
SELECT instance_name, [object_name],
[counter_name],
[cntr_value] FROM sys.dm_os_performance_counters WHERE [object_name] LIKE ‘%Database Replica%’
CY - Technical Memory extension 