I had a perfectly working Ubuntu Server system with U12.4 and Server installed and was setting up the mail system. When I was absent from my computer room my cat (furry type) apparently got to play with a USB dongle on an extension cable and removed it. The system went berserk and now will not boot. I ran Ubuntu 12.4 on the install disk and the directory structure and every thing appears to still be on the drives. How do I recover the GRUB boot section without the system repartitioning everything and wiping everything else? Please I am a total newb when it comes to Linux errors and fixing them.