So, when you do not have the possibility to enable HA (for whatever reason) you can do the following to make the VMs available in XenCenter again:
1. Locate the VMs which were running on the failed host with the following command:
xe vm-list resident-on=<UUID of the XenServer host> --multiple
You can determine the UUID of the host which failed by running the `xe host-list` command.
2. reset the power status of the VMs to halted using the following command:
xe vm-reset-powerstate vm=<Name of VM received from the command in step 1> force=true (repeat this step for all VMs which were running on the failed host)
PLEASE NOTE: make 100% sure that the VM you reset to halted using the vm-reset-powerstate command is actually powered off (e.g. because it was running on a XenServer which really failed) and not running on any other XenServer. Do NOT use this command while simulating the failure of a XenServer by stopping only the network of the host.