Abrir consola, ser root o usar sudo:

grub> root (hd0,1)

grub> setup (hd0)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd0)"... 17 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 (hd0) (hd0)1+17 p (hd0,1)/boot/grub/stage2
/boot/grub/menu.lst"... succeeded
Done.

grub>

ver Artículo Reinstalar GRUB