Resetting the db_owner role membership for master user

If you lock your master user out of the db_owner role membership on your RDS for SQL Server database and no other database user can grant the membership, you can restore lost membership by modifying the DB instance master user password.

By changing the DB instance master user password, RDS grants the db_owner membership to the databases in the DB instance that might have been accidentally revoked. You can change the DB instance password by using the Amazon RDS console, the AWS CLI command modify-db-instance, or by using the ModifyDBInstance API operation. For more information about modifying a DB instance, see Modifying an Amazon RDS DB instance.