Grub2 aka 'grub disaster'. Whatever you do procede with caution, (No backups to revert to an earlier MBR/installation that would get you back to the previous kernel I suppose?)
I had similar problems with a LinuxMint 9 installation where trying to update the kernel broke grub2. Maybe the previous kernel is still on the disk. As GnuTux you need to look at your /boot/grub/grub.cfg file (the one that says DO NOT EDIT, which of course you can)
Some possible tools.
It has been some time since I tried an original Ubuntu, does the live CD/DVD have an option for fixing grub.
There is a grub rescue disk 'supergrub2'
http://www.supergrubdisk.org/ free, small (fd size), live CD , worth a try. It should get you into the actual installation instead of working off a live CD/usb.
( did not have much luck with rescantux which is a bigger beast, its the same site, that might be worth a try as well).