- boot the system with a live usb
- make sure that the system hard drive is mounted. Find out what its block device name is and on what folder it is mounted. For this you might want to use the "df" command.
- make yourself the super user by issuing the command "sudo su -"
- use the mount command to mount the /dev and /proc mounts to "mountpoint"/dev and "mountpoint"/proc
- chroot into "mountpoint"
- run the following command "grub-install "blockdevname"
Let's assume that your system is booted via live usb, and the partition on which you have your operating system is /dev/sda3, and that partition is mounted as /mnt/hdsystem, "blockdevname" is /dev/sda (note that it is /dev/sda, it is NOT!!! /dev/sda3!!!), and "mountpoint" is /mnt/hdsystem.