From time to time you may have a VM which is not responsive in both the guest OS but also that it wont shutdown/restart on the hyper-v host AND it can also be in a stopped starte which really wont let you manage the vm anymore.
A running VM is just a little process on the host called vmwp.exe. There is one such process for each VM and to find the correct GUID (the unique identificatior) you will have to go into your folder where your virtual machine is located. The folder name and/or the name of the .xml file with the configuration is named with this GUID. Then you can just open task manager, locate the vmwp.exe process and the corresponding GUID; right click and end process. The virtual machines is now ok to be restarted again.