Why fresh install? If you copied ALL of the root partition, you only copy it back to /dev/sda1 (first partition on first disk), and it should be fine. But only if you have the whole root in the backup.
You will have to work from the live cd in live mode, because that allows you to boot the computer/server in live mode and work on it.
1. To get the UUID of the new /dev/sda1:
2. After you copy the files to sda1, to edit fstab with the new UUID:
Replace the UUID string in the / line with the new UUID of sda1. Save and close the file.
sudo mount /dev/sda1 /mnt
gksu gedit /mnt/etc/fstab (that's for GUI, for editing in terminal you can also use: sudo nano /mnt/etc/fstab)
3. For chroot to try updating grub2, if you mounted sda1 at /mnt as I wrote above, continue in terminal:
Now you are like inside the installation:
sudo mount --bind /proc /mnt/proc
sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo chroot /mnt
Exit chroot and unmount all:
update-grub (inside chroot you are like root so you don't need sudo in front of commands)
grub-install /dev/sda (to add grub2 to sda MBR)
Reboot and keep fingers crossed.
sudo umount /mnt/sys
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt