Windows 8 no longer boots after Grub wiped out the Windows Bootsector

Solution 1:

Somehow Grub has overwritten your Windows boot sector on your Windows 8 boot partition /dev/sda1. You will need to repair it in order to boot Windows again. And then don't use your custom Windows 8 grub entry (that is pointing at /dev/sda2) - the default on /dev/sda1 is required.

sda1: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Grub2 (v1.99)
    Boot sector info:  Grub2 (v1.99) is installed in the boot sector of sda1
                       and looks at sector 667401296 of the same hard drive
                       for core.img. core.img is at this location and looks
                       in partition 72 for . No errors found in the Boot
                       Parameter Block.
    Operating System:  
    Boot files:        /bootmgr /Boot/BCD

You can repair in one of two ways:

1. Boot a Windows repair CD to a repair prompt and for Windows Vista/7/8 run: bootrec /fixboot and for Windows XP run fixboot
2. Recover the backup boot sector using testdisk. The beauty of this solution is that you can do it from Ubuntu. Reference here.

Instructions for using testdisk

First install and run testdisk:

sudo apt-get install testdisk
sudo testdisk

Then follow instructions for each screen in testdisk:

First   screen:  Select "No Log" and press enter.
Second  screen:  Select /dev/sda - the hard drive containing the Windows system partition and  choose "proceed".
Third   screen:  "intel"
Fourth  screen:  "advanced",
Fifth   screen:  Select /dev/sda1 - the Windows system partition and choose "boot"
Sixth   screen:  "BackupBS"
Seventh screen:  type "Y" to confirm

Please also refer to this bug report and click "This bug affects me" so that Grub can be fixed.

If grub is only in the Windows boot sector, i.e. the Windows bootloader is still in the Drive MBR, then it's best to boot Ubuntu and install Grub to the MBR before fixing the Windows bootsector. e.g. sudo grub-install /dev/sda (adjust to the correct drive if not /dev/sda).

If you don't do this, after repairing the Windows boot sector, only Windows will boot, and you will need to install the Grub bootloader to the disk MBR in order to boot Ubuntu. Or you could use something like easyBCD. But to use grub, boot an Ubuntu live CD/USB (same release is best) and run (assuming your Ubuntu install is on /dev/sda5):

sudo mount /dev/sda5 /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda

Then reboot.