What to do when your Always On cluster loses quorum?
AGs are based on Windows Clustering. The WSFC procedures for quorum loss apply.


WSFC Disaster Recovery through Forced Quorum 
Force a WSFC Cluster to Start Without a Quorum


Once the WSFC is running, you can then force AG, if needed. Perform a Forced Manual Failover of an Availability Group:


  After forcing quorum on the WSFC cluster (forced quorum), you need to force failover each availability group (with possible data loss). Forcing failover is required because the real state of the WSFC cluster values might have been lost. However, you can avoid data loss, if are able to force failover on the server instance that was hosting the replica that was the primary replica before you forced quorum or to a secondary replica that was synchronized before you forced quorum. For more information, see Potential Ways to Avoid Data Loss After Quorum is Forced.