After crashing your system and getting entertaining colored screens on boot, you may have to consider booting in live mode from a bootable device such as an installation DVD or a USB-device. However you'll need to be able to log in as root in most cases to make changes in configuration files and others to get your system running again. While you can log in as root with a password on your normal system, you can do so in live mode by entering
sudo su -
into the terminal. This logs you in as root so you get to edit your important broken files to be able to boot from your hard drive again.