I got the message when I tried to run one of my VMs and got this or similar message about 64bit support:
VT-x/AMD-V hardware acceleration is not available on your system. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot.
Turns out it’s something to do with Hypervisor on Windows 8.1!
Turning it off again and doing the restart dance fixed it.