My Hyper-V VM disappeared
Solution 1:
I got a great solution from a Microsoft tech. The VM files were there even though Hyper-V didn't see them. So we first tried importing them. That failed.
But then she had a great idea - create a new VM, but for the hard drive, use the existing vhd file. That worked.
And the key thing on recovery is not the VM itself, that was easy to recreate. It's the actual content ont hat VM and the vhd had all that. So the Windows instance on the vhd booted right up and all was good.
So short answer, if your VM disappears, if you still have the vhd file and it's good, create a new VM and use that vhd.
Default location for the Hyper V files is "C:\ProgramData\Microsoft\Windows\Hyper-V" for VM config files and "C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks" for Virtual disks.