If the 'Host' field (on which the VM runs) is not empty, and assuming this host is disconnected, you can confirm 'Host has been Rebooted' in the Hosts view. There were issues in the past that lead to having VMs in 'Unknown' status and empty 'run_on_vds' property (i.e., the aforementioned 'Host' field in the UI) - in this case, which shouldn't happen anymore so if it happens for you with oVirt 4.5 please file an issue [1], you'll need to change the vm properties in the database - either by setting 'run_on_vds' to an accessible host or 'status' to 0.

[1] https://github.com/oVirt/ovirt-engine/issues