You’re running a Hyper-V on a Server with 2 disks and using Windows Software RAID-1. Your primary disk fails and your windows crashes, but on the reboot you have an option to boot from a secondary “plex” disk. If you choose the secondary disk as it’s your only option to boot now, the boot configuration setting (kernel) of windows is not identical with primary disk. Hyper-V will not start because the hypervisor was not enabled in the boot configuration file. What you need to do is run this command:
bcdedit /set hypervisorlaunchtype auto
This will edit the boot config file and enable the hypervisor. You have to restart your server and don’t forget to choose the Secondary disk to boot from again.