Windows Server 2008 R2 failed to boot with error message “bootmgr is missing”

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嘗試使用bootrec /fixboot 指令修復問題，修復後您還需要執行 "bcdedit /set hypervisorlaunchtype auto" 將 Hypervisor 設定成自動啟動。

Hyper-V virtual machines do not start after using Startup Repair
https://blogs.msdn.com/b/virtual_pc_guy/archive/2010/01/19/hyper-v-virtual-machines-do-not-start-after-using-startup-repair.aspx

Checked the boot order into the BIOS
Enter WinPE mode and try to repair with commands
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a. Insert a Windows Server 2008 R2 installation media, and boot from the media
b. At the initial "Install Windows" prompt windows click "Next"
c. DO NOT click "Install Now", click on the "Repair your computer" link located at the lower left corner of the "Install Windows" prompt window
d. Choose the repair tool option and click next
e. Select "Command Prompt"
f. Run the following commands to check if active partition is correctly set
    diskpart
select disk 0
select part 1
detail part
g. You should see the selected partition has a drive letter "C", has 100MB in size and that Active is set to "Yes"
h. exit diskpart
i. copy bootmgr from X:windowsbootpcat to c: (c drive here is the 100MB system reserved partition), reboot and check status
j. Run the following command, reboot and check status
    bootrec /fixboot
k. Run the following command, reboot and check status
    bootrec /rebuildBCD