grub mbr reinstall

Assuming that the "failed" system has / partition at sda1

1. mount /dev/sda1 /mnt/sda1
2. mount -o bind /dev /mnt/sda1/dev
3. chroot /mnt/sda1

(if necessary)
1. chroot /mnt/sda1 bash
2. grub-install /dev/sda1

Since UUID of partition _may_ have changed when you formatted it: edit

1. /etc/fstab
2. /boot/grub/menu.lst

Find the UUID
vol_id -u device