How to recover a database from a failed Availability Group?
The first attempt would be to try to get either the primary replica back online, or to get the synchronized secondary replica online to ensure that there is no data loss.  But provided this is a true disaster recovery scenario (as your original question implies), then it would seem as though your actual cluster (WSFC) has failed.  With the database entering the recovery_pending state, you can't unjoin the database to recover it.  Likewise, you would need the cluster in order to do a failover.  So the following assumes that you can't force quorum on either of the sync replicas, and you need to failover to the async.

The next step at this point to recover to the async secondary that is currently online, would be to force quorum (WSFC) and then perform a forced failover (with the potential for data loss).

For more information, I highly recommend you read up on BOL regarding the topic Perform a Forced Manual Failover of an Availability Group