I'm running 8.1 Pro and this error came long with zero effective fixes.
Finally I came across this:
From an elevated command prompt:
net localgroup administrators LOCAL /delete
net localgroup administrators SERVICE /delete
Afterwards, you'll get a "could not create or access saved state file" error along with the initialize error. When trying to run a VM.
Note the VM ID, goto C:\ProgramData\Microsoft\Windows\Hyper-V\Virtual Machines and in the folder with the same name as the VM ID, delete the .bin and .vsv file. - No VM's should be running at this point.
Launching the VM you'll see that your VM will be back up.
In my case, Hyper-V had been running fine all along and then after month or two of it not being used, I got the inexplicable error. This fixed it.