Q. I'm trying to start a Hyper-V virtual machine but it says the hypervisor isn't running. What can I do?

A. Make sure Hardware Virtualization and Execute Disable are enabled in the BIOS. Also check that the hypervisor is set to load. Run

bcdedit /enum

in a command prompt and look for hypervisorlaunchtype to be set to Auto, as shown here.

Windows Boot Loader
-------------------
identifier              \{current\}
device                  partition=C:
path                    Windowssystem32winload.exe
description             Windows 2008 R2
locale                  en-US
inherit                 \{bootloadersettings\}
recoverysequence        \{fc51eaa2-1dd2-11df-b10e-9dda4f16aaf7\}
recoveryenabled         Yes
osdevice                partition=C:
systemroot              Windows
resumeobject            \{fc51eaa0-1dd2-11df-b10e-9dda4f16aaf7\}
nx                      OptOut
hypervisorlaunchtype    Auto

If you've check those, the next thing to try is to shut down the machine, wait 30 seconds, then power it back on. Don't just do a restart, actually shut it down.

Next, check some of your more advanced BIOS settings. You need Hardware Virtualization and Execute Disable set to On. There are other options on some newer machines that can cause problems, commonly VT for Direct I/O Access and Trusted Execution. Try disabling VT for Direct I/O Access and Trusted Execution then shut down the machine, go into the BIOS again, and re-enable VT for Direct I/O Access but don't enable Trusted Execution. This has fixed several machines I've seen with these newer capabilities around hardware virtualization.