Boot your computer from a Live USB/CD/DVD
Note: only use a Live USB/CD/DVD of the Ubuntu version that you wish to repair.
However I've used Xubuntu 12.04 to repair 14.04 version
Find a root partition where your Ubuntu is installed
sudo fdisk -l
sudo parted -l
Run the following command into your teminal
sudo mount /dev/sdaX /mnt
Where X is number of partition where your Ubuntu is installed
For example:
sudo mount /dev/sda1 /mnt
This mounts the Ubuntu root partition on the hard drive
Run the following command to install Grub
sudo grub-install /dev/sda --root-directory=/mnt
Done
Reboot your machine
sudo reboot
You should see now grub menu now with your installation, so just login your Ubuntu now and run the following:
sudo update-grub
It will update your grub if you have other OS installed
sudo reboot
Done!… Read the rest