Boot to live USB
Mount the partition we want to restore grub (assume /dev/sda1)
sudo mount /dev/sda1 /mnt
# If /boot folder in another partition (assume /dev/sda2)
# you need to mount it too
# sudo mount /dev/sda2 /mnt/boot
Boot to live USB
Mount the partition we want to restore grub (assume /dev/sda1)
sudo mount /dev/sda1 /mnt
# If /boot folder in another partition (assume /dev/sda2)
# you need to mount it too
# sudo mount /dev/sda2 /mnt/boot