If faced with a grub prompt, I did these commands to fix it:

set root=(hd0,1)
linux /vmlinuz root=/dev/sda1 ro
initrd /initrd.img
boot

after reboot:

sudo grub-install /dev/sda
sudo update-grub

Found in a comment here: http://aaron-kelley.net/blog/2011/04/grub-prompt-after-upgrade-to-ubuntu...