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...

I have used Webmin before as a better replacement to cPanel and the other web hosting control panels. Not just that it is free, but it's ease of use.

So I decided to install it on my mini-mac today. I have learned so much about my own system in 20 minutes! More than in the last 8 months I have owned the mini.

I would strongly recommend this administration front end to any one who has a computer. It works on almost all of them it seems. Requires only perl be installed.