My PC just got upgraded to the latest Windows 10 Insiders build (slow ring, build 10565) and suddenly a couple of VM's were missing from the Hyper-V Manager.
I first suspected the security settings on the directories were the problem, but my changes didn't fix anything. After browsing around the internet, I found a couple of fixes that might help you if you have the same problem.
Save your machine
The post Windows 10 Preview - VMs Missing In Hyper-V Manager by Rhoderick Milne shows he was still able to see the Virtual Machines using PowerShell.
He got the Virtual Machines to show up again after saving them from the command line:
Get-VM | Where { $_.State –eq "Running" } | Save-VM
Get a heartbeat
Although he could see them again in the Hyper-V Manager, some were stuck when starting up.
The problem is the integration service is reporting a corrupt heartbeat. He fixed this by disabling the heartbeat service for these machines.
Get-VM | Where { $_.Heartbeat -eq "OkApplicationsUnknown" } | Disable-VMIntegrationService Heartbeat
Adjust the registry
The command was not sufficient for my situation. The VM's were still missing.
Then I ran across the article PSA: Missing Hyper-V VMs on Windows 10 - Build 10547 by Ben Armstrong.
He indicates that it's a registry problem in this build of Windows.
Set-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization" -Name "CurrentVmVersion" -Value "6.2" Restart-Service VMMS
Conclusion
This fixed it for me.
Three Simple PowerShell statements can rescue your VM's from oblivion on the Windows 10 preview.