AAG Database is in “Not Synchronizing / Suspect” mode due to a system Assertion Error

User has reported that, AAG Database is in “Not Synchronizing / Suspect” mode

From SQL Server Error logs, we found that, the Database “XYZ” is in “Not Synchronizing / Suspect” mode due to a system Assertion Error on primary replica.

Tried to remove this database from AG but no luck , its HUNG with the below AAG Internal Async work Wait Type.

Recycled all replicas under the AAG.

After the instance restarts, we brought the database online & added it to AAG. 

Also, we verified that there was no data corruption. Hence NO data loss.

If you having Not Synchronizing on secondary replica's, RESUME will fix this issue :)