This applies to the situation that when Windows and Ubuntu coexist on a computer and we reinstall Windows for some reasons, the Grub loader is lost. However, Ubuntu is still there. Do the following to recover.

  1. Restart with Ubuntu LiveCD.
  2. sudo grub
  3. Use find /boot/grub/stage1 to find where the Grub is installed. For example (hd0,6).
  4. root (hd0,6)
  5. setup (hd0)
  6. quit
  7. Restart the computer, and it’s done!

Update: with the introduction of Grub 2, the above method is not working. See here for a solution, adapted for my case below.

  1. Use gparted to figure out where the system has been installed. For my case, / is at /dev/sda11, and /home is at /dev/sda9.
  2. Mount the root directory. mount /dev/sda11 /mnt
  3. Install the Grub 2 loader. grub-install --root-directory=/mnt/ /dev/sda
  4. Restart the system and the Grub 2 loader is shown.
  5. Use the following to boot the system. 
    grub> find /boot/grub/core.img
grub> root (hdx,y)   (previous command will output the x,y)
grub> kernel /boot/grub/core.img
grub> boot
  6. After the system is booted, install the bootloader. sudo grub-install /dev/sda