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Have you tried repairing your boot loader? If you're using UEFI(which I would guess):
source
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to recover the loader (BCD) configuration, you have to boot from the original installation Windows 8 DVD (or a recovery disk or a special EFI bootable flash drive) and open the command line choosingSystem Restore – > Troubleshoot-> Command Prompt or pressing Shift+F10).
Start diskpart:
[TABLE="width: 712"]
[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]diskpart[/TD]
[/TR]
[/TABLE]
Display the list of disks in the system:
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]list disk[/TD]
[/TR]
[/TABLE]
Select the disk with Windows 8 installed (if there is one disk in the system, it will have zero index):
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]sel disk 0[/TD]
[/TR]
[/TABLE]
Display the list of volumes in the system:
[TABLE="width: 712"]
[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]list vol[/TD]
[/TR]
[/TABLE]
In this example, you can see that the EFI volume (it can easily be recognized by its size of 100 MB and FAT32 file system) has the index volume 1, and the boot volume with Windows 8 installed is volume 3.
Assign any disk letter to the EFI volume:
[TABLE="width: 712"]
[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]select volume 1[/TD]
[/TR]
[/TABLE]
[TABLE="width: 712"]
[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]assign letter K:[/TD]
[/TR]
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Close diskpart:
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]exit[/TD]
[/TR]
[/TABLE]
Go to the bootloader directory in the hidden volume
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]cd /d k:\efi\microsoft\boot\[/TD]
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[/TABLE]
Recreate the boot sector on the boot partition
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]bootrec /fixboot[/TD]
[/TR]
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Delete the current BCD configuration file by renaming it (save the older configuration as a backup):
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]ren BCD BCD.bak[/TD]
[/TR]
[/TABLE]
With bcdboot.exe, create BCD store again by copying the boot files from the system directory:
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[TR]
[TD="class: line_numbers"]1[/TD]
[TD="class: code, bgcolor: #EEEEEE"]bcdboot C:\Windows /l en-us /s k: /f ALL[/TD]
[/TR]
[/TABLE]
where C:\Windows – is the path to the directory with Windows 8 installed.
/f ALL – means that the boot files have to be copied including those for UEFI and BIOS computers (potential ability to boot in EFI and BIOS systems)
/l en-us – is a type of the system locale. By default, en-us – English (USA) is used.
Now you have to restart your computer. Then in the list of bootable devices there appears Windows Boot Manager where you can choose desired operating system to start.