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 happen 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.
This condition happens when the esx host cannot access the vmx file (vmware configuration file) in the datastore, so VMWare just sort of forgets the configuration of the machine. The process which runs the virtual machine is still running on the host, so your VM does not crash. But, you still need to recover since HA, DRS, etc… no longer work when the VM is in this state.