99% sure that the problem is with a Kernel update. Basically you installed a new brain to ubuntu but the update process forgot to tell the body to connect to it! :-/
If this applies to you, there is bug report on launchpad. Bug comment #3 and #7 seem to contain a workaround for manually booting the old version of the kernel.
I would add - to that thread - that if you manage to boot ubuntu with the old kernel, you could try to issue the command
which should find out the new kernel and rebuild the normal Grub menu.
You should really visit the launchpad bug to check out if progress being made and new (hopefully easier workarounds), however, as of 27/11/09 16.25 GMT the following is the most complete solution provided there.
At the grub prompt enter:
linux /boot/vmlinuz-2.6.31-14-generic root=/dev/sda1 loop=/ubuntu/disks/root.disk ro quiet splash
(if it doesn't work, change sda1 to sda2 or sda3 or ..., and please don't load kernel ...-15-...!)