Linux...<Username> is not in the sudoers file. This incident will be reported?

Reboot the computer. Recovery mode or single user mode should be a choice in the boot menu. You might have to press Escape when you see the word "grub" or the first splash screen. Or you could boot from the CD. By the way, don't worry about being reported. When you finally get to edit the sudoers file, one of the options is to insult a user who doesn't have the proper username or pword.