What you need to do is install GRUB 2. To do this you need to boot to the live instance, mount your root partition and install.
From a live instance, find the partition on which your root partition is loaded. GParted will tell you this, or you could use
sudo fdisk -l
Go for the partition in which ubuntu is installed.
Once you have your partition you need to mount it. Assuming the root partition is on /dev/sda5, that’d be:
sudo mount /dev/sda5 /mnt
Then install GRUB 2
sudo grub-install /dev/sda --root-directory=/mnt
By: J Morrissey