The easiest way to restore a broken system’s GRUB2 files is simply to replace them. This works with Ubuntu 9.10 or later only and only if the system uses GRUB2 (which it does if it was originally installed using 9.10 or later, but might not if Ubuntu was originally installed using an older Ubuntu release and upgraded as GRUB2 is not automatically installed during release upgrades). In any case, your best bet is to use a DVD from the same release (such as 13.10) as what you have installed on the hard drive.
To get started, boot using the live DVD and open a terminal from the Dash. Then determine which of the hard drive’s partitions holds the Ubuntu installation, which you can discover using the following: