You have to press one of these keys F2, F12, Delete, Esc during the Boot Splash to pick which drive you want to boot.
The other reasons are that you do not have a bootable disk or the drive is bad. If you are using an USB CD Drive you have to be able to boot from USB.
To make a long story short, I know the drive works because the CD shows up once booted, and I know the computer can boot from CD because it's done it before. It just suddenly stopped working. CD is set to first boot priority in the BIOS, and instead of booting from the CD, the laptop starts booting the OS (Linux Mint KDE 9) right away. Does anyone know how to make this computer boot from CD?