Note to self: Always make a copy of your boot sectors using dd first. 
Note to world: These commands will only work if you have a separate boot partition. 



# take note of your Linux partitions (/dev/sde3 == /, /dev/sde1 = /boot in my case)

fdisk -l 

# Mount Linux partitions using your live installer cd / usb key: 

mount /dev/sde3 /mnt
mount /dev/sde1 /mnt/boot

# re-install grub to /dev/sda

grub-install --boot-directory=/mnt/boot /dev/sda 

Source(s):
https://help.ubuntu.com/community/Grub2/Installing