Sometimes virtual machines within your VMWare environment may show up as invalid. The machine is in-fact still running at this point; but you are unable to manage the virtual machine. This can handle for a few reasons, but in my experience the most common is when the esx host is unable to access the storage.
I have seen this caused by high latency when accessing an NFS datastore, when you leave an offline datastore mounted for an extended period, and I have also seen this happen when a SAN controller failover event occurs.
