https://neosmart.net/wiki/recovering-windows-bootloader/

Let’s start by telling the recovery console to fix our MBR and bootsectors:

bootrec.exe /fixmbr
bootsect.exe /nt60 all /force

Then ask it (nicely!) to try and rebuild your BCD data from scratch:

bootrec.exe /rebuildbcd

If you’re lucky, this’ll work and you’ll see a message telling you everything went OK.

Reboot your PC and try booting back into Windows to see if it worked. Don’t forget to remove your Windows Vista DVD or Recovery DVD from the drive, or you’ll end back in the repair center!

If your PC still doesn’t boot, read on to Attempt Three for a more powerful recovery option.
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The first order of business is to make sure the MBR and bootsector contain the right references to the Windows bootloader:

bootrec.exe /fixmbr
bootsect.exe /nt60 all /force

Now we get rid of the old BCD registry, and create a new one instead.

Note: We’re assuming that the boot drive is drive C: below. If your computer is configured differently, be sure to use the appropriate drive letter here instead.

attrib -h -s C:\boot\BCD

del C:\boot\BCD
bcdedit /createstore c:\boot\bcd.temp
bcdedit.exe /store c:\boot\bcd.temp /create {bootmgr} /d "Windows Boot Manager"
bcdedit.exe /import c:\boot\bcd.temp
bcdedit.exe /set {bootmgr} device partition=C:
bcdedit.exe /timeout 10
attrib -h -s C:\boot\bcd.temp
del c:\boot\bcd.temp

The first two commands may fail. Assuming you entered them correctly, that is fine; it simply indicates the BCD does not yet exist.

Now we have a clean, working Vista bootloader. But we need to add a Windows entry to it:

bcdedit.exe /create /d "Microsoft Windows" /application osloader

bcdedit.exe should return a message with a GUID for the newly-created entry, something like this:
The entry {c0dfc4fa-cb21-11dc-81bf-005056c00008} was successfully created.

You’ll need to use the value that bcdedit.exe returned for you below, along with the drive letter for the drive that Windows is installed to:

Again, make sure to replace C: with whatever the correct drive is for your Windows Vista/7/8 installation.

bcdedit.exe /set {c0dfc4fa-cb21-11dc-81bf-005056c00008} device partition=C:
bcdedit.exe /set {c0dfc4fa-cb21-11dc-81bf-005056c00008} osdevice partition=C:
bcdedit.exe /set {c0dfc4fa-cb21-11dc-81bf-005056c00008} path \Windows\system32\winload.exe
bcdedit.exe /set {c0dfc4fa-cb21-11dc-81bf-005056c00008} systemroot \Windows

And, last of all, tell the bootmgr bootloader to list the new entry or else it’ll remain hidden:

bcdedit.exe /displayorder {c0dfc4fa-cb21-11dc-81bf-005056c00008}

Now your bootloader has been completely removed and rebuilt from scratch. If you got this to work, feel free to consider yourself one of the finest graduates of the NeoSmart Technologies Institute of BCD Learning.

At this point, you have a clean, untouched, and (hopefully) fully-working bootloader with one entry for Windows.

Go ahead and reboot your PC, get back into Windows, and use EasyBCD to further tweak the bootloader to your heart’s content.

We advise backing up the BCD registry immediately with EasyBCD before making any changes.