Our hosts decided to reboot. Only one of the hosts wouldn’t start VMs. They’d sit there fine while off but attempt to start them and they’d fail.

“Virtual machine ‘VM Name’ could not be started because hypervisor is not running.”

Turns out this is because of Windows update

KB5009624

KB5009595

Are the cause. Uninstall and all will be fine.

wusa /uninstall /kb:5009595 /quiet /norestart

wusa /uninstall /kb:5009624 /quiet /norestart

You’ll probably need to do a restart but you might want to control it rather than the command doing it.

Probably a good idea to remove from WSUS for now if still using WSUS.