Sometimes you might find that one of your nodes and all its containers, VMs, and storages have gray question marks beside them. This happens because the cluster has lost quorum.
When this happens, sometimes you can even still see the real-time stats in the summary page the node and machines still ticking along.
Here’s the command to fix it:
service pve-cluster stop && service corosync stop && service pvestatd stop && service pveproxy stop && service pvedaemon stop
service pve-cluster start && service corosync start && service pvestatd start && service pveproxy start