After recently cloning a Hyper V machine after another RAID issue I was presented with an issue where none of the guest would start after a reboot.

This was strange as before the clone the machines started and ran fine. When attempting to start one of the guests I was presented with the error.

Virtual Machine could not be started because the hypervisor is not running

Performed some research on the internet which said that this error can occur when DEP and the Virtualisation settings for the processor are not enabled in the BIOS. So I reset the machine and checked those settings, they were all set correctly. (KB 2490458)

Given that they were all correct I decided to disable the services boot into windows and then rebooted and re-enabled the settings. The error remained so the issue wasn’t hardware.

After more research I found that I may need to set the hypervisor to start automatically using bcdedit (Boot Configuration Data) 

So I ran the command

bcdedit /set hypervisorlaunchtype Auto

and restarted the machine. Once the machine restarted the guests were able to be started and ran fine.