VMWare ESXi 5.0 U1
It happened to me that I created a new VM, then copied over a template VMDK but forgot to add it. When I started the VM it complained of no OS, so I edited the config and added the HDD.
Right after this operation, the VM became gray and was no longer manageable.
I removed it from inventory, then added it back again and now it was showing properly as powered on.
I powered it off, then on again.
Now, the VM kept showing a text cursor “dancing” on the screen like crazy.
I powered it off, removed the HDD, then added it back and powered it on.
Now, it’s usable.