Hyper-V Virtual Machines Fail to Start and Initialize after Windows 10 Upgrade from Windows 8.1

UPDATE: 

I never was able to complete all the troubleshooting I wanted to because somewhere along the line my Windows 10 installation became corrupted. Windows would boot and get to the point where the sound driver loaded -- I could hear the speakers energize -- and the mouse became useable. But booting stopped at the black screen just prior to where the sign-in screen should have appeared.  My troubleshooting included needing to run bcdedit and ensuring the entry hypervisorlaunchtype was set to Auto (it had been set to Off).  This allowed Hyper-V to launch properly correcting the first problem.  However, no matter what I did, existing VMs -- even a newly-created VM -- would not launch.  I tried various things with permissions and other troubleshooting which led to the corrupt Windows. 

So I bit the bullet and did a clean Windows 10 install on a new SSD drive. (I had to do an upgrade using my old Windows 8 activation key since I didn’t have a new Windows 10 key.) Hyper-V works correctly in all aspects. (I have an  I was eventually able to add the old drive from the corrupted Windows image and retrieve all of my data, including the Hyper-V virtual drive files. I am able to attach them to the new virtual machines and they work great.

The one last bit of troubleshooting I wanted to perform (but couldn't because of the Windows corruption on the old installation) was to uninstall all Hyper-V services, delete the C:\ProgramData\Microsoft\Windows\Hyper-V folder and subfolders, reinstall Hyper-V services letting it recreate the tree, create VMs, and attach the old VM dive files to them. I really think this process might have worked but I'll never know.

perhaps you could run those steps with me? my windows 10 does not appear to be corrupted (everything except Hyper-V works). I changed the setting of the VM I was testing with to start at launch as described in your post. I will try the part you never got to try.

One thing I noticed, is the locations where the VM's are kept is normally unaccessible by anything gut Hyper-V. is this the same as your system?

also I found a TechNet forum thread for server 2012 that said dx11 hardware was necessary for remoteFX to function. windows 10 pro has no options for changing whether the VM uses RemoteFX or not. even though my laptop was supposed to be a Windows 7 laptop, the video card installed by Toshiba was a DX10 card and only supports limited DX10 features. Hyper-V works for me on Windows 8.1.2, but not Windows 10 Pro. If anybody know how to change the RemoteFX options in Hyper-V on Windows 10 Pro, as in turning off RemoteFX, please speak up.

After getting the hypervisor to start (the bcdedit I wrote about earlier) I was still getting event IDs 3040 and 15130 errors.  These pointed to permissions issues of the VM folders after all other mentioned issues were confirmed as not being the probable causes.

The location for the Hyper-V files (other than the virtual hard drive VHDX files) is c:\ProgramData\Microsoft\Windows\Hyper-V.  This is a normally hidden folder and you usually should be able to see it and then attempt to delete it.  When attempting to open it I get the "You don't currently have permission to access this folder" but as an administrator you can click yes to get that access.  The virtual hard drive files are normally located in the c:\users\Public Documents\Documents. 

Note:  Before you start troubleshooting, if possible, create a full image of your hard drive in case something really breaks and you need to recreate you working Windows 10.  I wish I had done this as it would have allowed me to go back to start rather than having to abandon the broken installation and reinstall Windows on an alternate device. 

1. Just in case there is a problem along the way, you should backup and protect any of the VHDX files as these contain the actual virtual machines.   Multiple copies of the files in alternate locates reduces the risk of corruption as you can always get another copy of the file.

2.  Check and take note of Hyper-V settings, especially things like those your VM needs (like the virtual switch). 

3. You should create a system restore point to allow you to roll-back if something catastrophic happens.
4. Turn off the four Hyper-V items in Windows Features.  The PC should reboot a couple of times. 
5. Delete the c:\ProgramData\Microsoft\Windows\Hyper-V folder.
6. Restart the computer.
7. R-enable the four Hyper-V Windows features.  The PC should reboot a couple of time.
8. Check to see if the c:\ProgramData\Microsoft\Windows\Hyper-V folder was created.  If so, proceed to launch the Hyper-V manager.  Set the setting for Virtual Switches, etc. to the same as before, e.g. create a new virtual switch (if you had one for the prior machines) using the same name as before.
9. Create a new VM.  Select the VHDX file , Virtual Switch (if you had one, etc.) used by your old VM as the hard drive for the new VM.

10.  Hold your breath and connect to the new VM and attempt to start it.  When I did this in my new Windows 10 installation (using steps 8 and 9) my machine started immediately. 

Let me know how it goes.

removed the hyper-v components through the add/remove windows features menu, but after rebooting, the hyper-v directories in c:\programdata, were already removed. the test VM's VHD was still present in public documents however. I did try mounting the VHD through disk management, and windows reported that it was an uninitialized disk. which I suppose is normal.

I did find this guide online for windows 10 released by MS which is nice, since all the other ones I could find were about windows 10 preview.

https://msdn.microsoft.com/virtualization/hyperv\_on\_windows/windows\_welcome

I'm running through the steps now and will report back on success or failure

removed the hyper-v components through the add/remove windows features menu, but after rebooting, the hyper-v directories in c:\programdata, were already removed. the test VM's VHD was still present in public documents however. I did try mounting the VHD through disk management, and windows reported that it was an uninitialized disk. which I suppose is normal.

I did find this guide online for windows 10 released by MS which is nice, since all the other ones I could find were about windows 10 preview.

https://msdn.microsoft.com/virtualization/hyperv\_on\_windows/windows\_welcome

I'm running through the steps now and will report back on success or failure

The guide in the link really doesn't say much -- if you've ever used Hyper-V before it contains processes you've already used.  

I presume the "steps" to which you refer are the ones I provided?