If you have a Live ISO, boot from that.
Once you have the terminal in that live ISO, open GParted to examine the partitions.
Once you have identified which device has your bootable system (/dev/sd?), enter the following commands at the terminal prompt:
sudo grub-install ${device_full_path} # i.e. /dev/sda
sudo update-grub
Then reboot. That should fix it.