grub rescue>  ls
(hd0,4) (hd0,3) (hd0,2) (hd0,1)

grub rescue>  ls (hd0,4)/boot
... some kind of 'not found' message

grub rescue>  ls (hd0,3)/boot
... some kind of 'not found' message

grub rescue>  ls (hd0,2)/boot
... grub ... initrd.img-2.6.32-33-generic ... vmlinuz-2.6.32-33-generic

grub rescue>  set root=(hd0,2)/boot
grub rescue>  insmod linux
grub rescue>  linux (hd0,2)/boot/vmlinuz-2.6.32-33-generic
grub rescue>  initrd (hd0,2)/boot/initrd.img-2.6.32-33-generic
grub rescue>  boot

busybox>  cd /boot
busybox>  mv mod/* grub
busybox>  reboot

sudo mount /dev/sda1 /mnt

sudo mount --bind /sys /mnt/sys

sudo mount --bind /proc /mnt/proc

sudo mount --bind /dev /mnt/dev

sudo chroot /mnt

sudo apt-get install --reinstall grub2