Sometimes it may happen that your vm has hanged and XenCenter controls(Reboot and Force Reboot) do not work.

You may see an error when you try to force a reboot (The operation could not be performed because a domain still exists for the specified VM.).

This is a process to reboot the hanged vm from the XenServer host console.

To get the uuid of the VM that is hung:

xe vm-list

Get the vm domain id by uuid:

list_domains

Then destroy the domain of the vm:

/opt/xensource/debug/destroy_domain -domid <domid>

Now you can force a reboot on your vm:

xe vm-reboot uuid=<uuid> --force

If it has no effect you can also try:

xe vm-reset-powerstate uuid=<uuid> --force