After doing a dist-upgrade to 11.04 a while back, my Kubuntu machine refused to boot.
I eventually tracked this down to GRUB2 os-prober feature freaking out and trying to boot off the partition that held only the "/usr" directory rather than the one with the root and "/boot" directories. (Why, I have no idea. The latter is the first partition and the one that holds the MBR, so os-prober is clearly going to some effort to find and enforce the wrong partition).
After searching on Google and with a bit of experimentation, I was able to fix it by booting off a LiveCD, adding the line "GRUB_DISABLE_OS_PROBER=true" to "/etc/default/grub" and then running "sudo update-grub".