This one actually happened to me one morning in class. nothing like having an your Hyper-V give you a brand new error right in front of your students. Considering I've been using Hyper-V for many months without issue, this was a surprise.
I discovered that the problem actually was because the server did not pick up the USB hard drive that I was using for this set of VMs. Once the server recognized the USB drive, I had to restart the Hyper-V Virtual Machine Management service.