Sometimes when you try to change your boot priority in your BIOS you cannot select the CD ROM option or sometimes it is not even available to choose.
Your CD ROM is dead!
This means you have a problem with your cd rom drive. They do die after a while so try plugging in another one in and see if that solves your problem. Dead CD roms might still show up in the BIOS but do not work.
You can easily replace a CD ROM with a new and improved drive. Most likely a DVD drive or even a BLURAY drive.
Your Cd ROM is not installed
You might not have your cd rom installed correctly. For example it might have a power cable plugged in but not an IDE or Sata connection. I have done this many times when building computers.
You can see an exclamation mark!
You can see and Exclamation mark next to the device you want to choose as the first boot device. For example you might see this ! next to your cd Rom or even a hard drive you have installed.
You chose to boot from CD
You actually chose to boot from CD but it still will not! Maybe the CD you want to boot to is not a bootable CD. Or maybe you did not save your changes in your BIOS on exit.