Cliff Hobbs at myITforum.com

It is possible under Hyper-V to get into a state where you have a virtual machine that is still running correctly - but does not show up in the virtual machine list under the Hyper-V management console.  The two most likely causes for this are:

1. The virtual machine management service (VMMS) is not running. 
   If this is the case you will not see any virtual machines (or interact with the server in general).  You can fix this by just starting the VMMS again (run "net start vmms" from an administrative command prompt).
2. The virtual machine management service cannot find the virtual machine configuration file.
   With Windows Server 2008 - if a virtual machine configuration file is stored on network based storage, and there is a momentary disruption in network connectivity - the virtual machine management service will stop listing the virtual machine in question, even though it is still running.  You can correct this problem by restarting the VMMS (run "net stop vmms" followed by "net start vmms" from an administrative command prompt).
   Note that this should not happen with Windows Server 2008 R2, as we have updated the VMMS so that it will recheck for the configuration in this case.

Cheers,
Ben