SQL Server FCI won't start after OS Upgrade
Prior to Windows Server 2012 R2, doing an in-place upgrade of Windows for an FCI is not supported. Per Microsoft's documentation: 


  Upgrading the Windows operating system of a failover cluster is not
  supported for operating systems before Windows Server 2012 R2. To
  upgrade a cluster node running on Windows Server 2012 R2, see 
  Cluster Operating System Rolling Upgrade.


Because your starting OS was Windows Server 2008 R2, you are unable to do an in-place upgrade to Windows Server 2012 R2. Instead, you will need to build a new Windows Cluster, do a fresh SQL install for a new FCI, then migrate to that new FCI.

You can do this using existing hardware by removing a node from the cluster, doing the fresh install of both Windows &amp; SQL, then repeating the process one node at a time to move servers from the old cluster to the new cluster.

Once you are upgraded to Windows Server 2012 R2, you will be able to do in-place OS upgrades in the future.

Given that you've already upgraded the OS, your best bet is going to be to uninstall SQL Server, and re-install the FCI from scratch.