Is the CD still readable? Did you tried to mount in other machines and tested it??
Did you also check the boot order?? Because a lot of time I've made sure and I sitll missed it. :
Other than that, I don't know. I've some CDs that I can read but they won't boot. My guess is that the data or the area that holds bootable info ( the 2.88 MB area ) got somehow corrupted.