Recovering Hyper-V virtual machines that have disappeared
This excellent article by CoolComputerGuy has saved me on more than one occasion. Hyper-V's virtual machine management service relies upon the XML file of each of the VMs in order to load them into the console correctly.
To cut a long story short, if one of the XML files becomes corrupt it means that the machine does not load into the console correctly.
Thanks to CoolComputerGuy. I hope it helps someone else who finds themselves in a similar situation.