Ubuntu is usually very stable (especially 12.04 LTS) and can boots on almost every one machine. But sometimes, there are cases when video driver crashed or not detected at all, or something other happened and X fails to start. This is the time you need to boot to text mode or so called run level 3.
Here is how: (warning: non-professional skills required to boot, high level of skills required to fix the things)
Just follow the next simple instructions:
1. reboot your Ubuntu or just power-off and power-on again.
2. In your GRUB boot screen select the first option and press key 'e'. Sometimes, if you have installed Ubuntu alone, there is no GRUB boot screen because there is no boot delay. In this case, you should find the way to edit the /etc/default/grub file and apply the changes.
3. Edit the line starting with linux as shown to the picture by replacing the words quiet splash with text
4. Boot by pressing F10 key
Now your Ubuntu should boots without graphical mode (called X server) and you could enter your username and password on the login screen and start fixing the things, like installing new drivers or something else.
All pictures are grabbed from my fresh Ubuntu 12.04 install on VirtualBox.
That's all. Enjoy.