The bad news is that Ubuntu triggered a bug in your BIOS. The good news is that it's easy to make the computer bootable again: all you need to do is shut the power off completely, i.e. disconnect it from wall power and remove the battery. Wait a few seconds, put the battery back, and you should be able to boot.

Figuring out the cause of the bug can be difficult. First, see if that's a one-off thing or if it's repeatable. At the Grub prompt, highlight the “Ubuntu …” line, press e, use the arrow keys to navigate to the linux /vmlinuz … line, delete the words quiet splash at the end of the linux, and press Ctrl+X to boot. If that was a freak event and your computer boots this time, it's not worth investigating further unless the bug occurs again. If the computer hangs, note the last few messages that appeared on-screen: they will be a good clue as to what is breaking. How to proceed from there depends on these messages.