In order to install a new operating system on a computer, you can make a bootable DVD that includes the software to install the new system, put it in the DVD/CD reader, and reboot your computer. If all goes well your computer will boot off the DVD/CD reader and then you follow the install process and there you go.
But sometimes this doesn’t work. The most common reason is that your computer is not configured to boot from the DVD/CD reader first (if it has a bootable disk in it). You have to go into bios and change the “boot order” so “boot from DVD” is on top of the list, above “hard drive.” A less common problem is that your motherboard is old and not configured properly at a deeper level, which may require installing a new bios. Another, probably common, reason is that the DVD you made is messed up somehow. The “.iso” image you downloaded is corrupted or something went wrong during the burning process. One way to check that is to use the checksum hashtag to verify the image. Never heard of that? Just look it up. It involves obtaining and comparing two numbers which are constructed from the image. One number is provided by the maker of the image using specific software that does this, the other number is obtained on your final version of the image (or downloaded version) using similar software. If the numbers are different, the data are corrupted. This might be more likely if your computer was doing wonky things while downloading or burning, or if the drive you did the burning with is messed up. (Yet another possibility is that the disk is dirty or damaged, but if you just made it, that seems unlikely.) A typical run down of the problems, in the context of installing Ubuntu Linux, is here.
But I think there is yet another explanation that occasionally happens. It is possible that your DVD/CD reader will only participate in the boot under certain circumstances. This would be the case either with older DVD/CD readers, or possibly, a matter of a broken or dirty DVD/CD reader. I’m pretty sure this is the case because I have an old computer with a DVD/CD drive into which I can put known functioning bootable CD’s and get results, but that will not boot off a known functioning bootable DVD.
I could have cleaned the DVD drive, or I could have replaced it (they are cheap). I did not do the former for no particularly good reason, and I did not do the latter because a long time ago I learned it was better, when buying a new DVD/CD drive, to get a nice external drive so it can be moved between computers.
What I did do in this case was to burn a regular CD rather than DVD with a system. This is a problem if you want to install Ubuntu because, apparently, there are no longer such images available for current versions of the operating system. But, Debian sill has an image that does this. Since I was giving serious thought to installing Debian rather than Ubuntu (which is based on Debian but with a lot of changes that I don’t like), this was a good move. Someday I’ll clean the DVD.
I am not certain that I’ve isolated an actual problem, but when I search around for explanations for what I observe, I tend to find the same thing over and over; the usual explanations are repeated and the user with the problem is left wondering. So, I’m putting this on the Internet for people to run into while searching for answers.