The NSX Controller cluster should have three members. If after troubleshooting, you determine that one of your NSX Controller appliances are unrecoverable, you can add an appliance to the cluster to replace it, or if needed, redeploy the NSX Controller cluster.
An NSX Controller cluster must have majority to function normally. If two out of three members are online, the cluster still has majority. You should restore the three-member cluster by bringing up the offline NSX Controller. If you cannot bring it up, you can add another NSX Controller appliance to replace it, and regain majority. See Replace a Member of the NSX Controller Cluster.
If one of out three members are online, the cluster does not have majority, and will not function normally. If you bring up the offline members, the cluster will regain majority. If you cannot bring up either of the offline members, you can redeploy the NSX Controller cluster. See Redeploy the NSX Controller Cluster.
Verify through troubleshooting that the appliances are not recoverable. For example, these steps may recover the appliances without having to replace them.
Verify that the appliances have network connectivity, and resolve if not.
Reboot the appliances.
What to do next
Get the NSX Controller cluster status. See Get the NSX Controller Cluster Status.