If unable to power off the virtual machines using SSH/RDP, terminate a virtual machine through the ESXi host SSH session refer to Unable to Power off a Virtual Machine in an ESXi host. Here are steps on How to register or add a Virtual Machine (VM) to the vSphere Inventory in vCenter Server. (After which the issue ESXi host can be rebooted).

Register the Virtual Machines to other stable hosts right after shutting down the Virtual Machines. You can reduce the downtime for the virtual machines prior to rebooting the 'Not Responding' ESXi host. Once done, using BMC/iDRAC Power Control, Power Cycle (reboot) the ESXi host to get the ESXi host to a stable state. If the DCUI console interface is stopped responding and you are unable to restart managements agents or services, manual shutdown of the virtual machines (registered on that node but showing disconnected on vCenter) using SSH or RDP is the only other way around to bring back the Virtual Machines and the ESXi host to a stable state. See to VMware KB - 1003490 for Restarting the Management agents in ESXi. However, if SSH is not enabled, accessing the BMC/iDRAC Port would give access to the ESXi DCUI Screen where the management services can be restarted.

Restarting the Management agent of an ESXi host could be done directly using a CLI/SSH session to the ESXi (if SSH is enabled prior to the issue). One other response to a 'Not Responding' ESXi host is by restarting the Management agents on the ESXi hosts. This can be done using an SSH session to the ESXi host. Restarting hostd or vpxa on the host could help bring back the host manageability on the vSphere Client.

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