As you mention you can boot from a live cd, you can reset the boot loader manually (which may be part of the problem.)
Boot to the live CD
Determine the block storage device (hard drive partition) of the old system's boot drive, i.e. /dev/sda3 or some such (this is the location of your /boot files in the old system.) By default, it's on the root partition, you can figure it out by doing in terminal:
sudo mkdir /recover
sudo mount /dev/sda2 /recover
This is assuming your root or boot drive is on /dev/sda2, it may be on /dev/sda1-?? or /dev/sdb? or /dev/sdc? ; it would really depend on your system. Assuming it's /dev/sda2...
mount /dev/sda2 /mnt
mount --bind /dev /mnt/dev
mount --bind /sys /mnt/sys
mount --bind /proc /mnt/proc
You're now chrooted into your existing system. Update the system to ensure that the upgrade worked.
then to be sure the boot system is working, run
(this assumes your on a single boot system with the disk device /dev/sda with just ubuntu, using grub2 as your boot manager.)