If a host cannot be managed via vCenter or from vSphere client, you can still use the CLI (on the host itself or via network using SSH or vSphere CLI/RCLI) to partially manage the VMs on the host.
For this to work you must have already enabled the local console, Tech Support Mode and/or SSH access:
Once logged in, you can use these commands to control the VMs. Note that there are differences between ESX and ESXi.
|List the registered VMs
|Get power state of VM
|vmware-vim-cmd vmsvc/power.getstate vmid
|vmware-cmd path-to-VMX-file getstate
|Power on a VM
|vmware-vim-cmd vmsvc/power.on vmid
|vmware-cmd path-to-VMX-file start