You're not giving much information, like even just how far it's getting into the boot. If you're not getting to the first prompt then I would guess an improper burn or even a bad md5 sum on what you downloaded. However, if you got to the first prompt (I'm guessing you did) then you could have different issues. While you could still have a bad CD (particularly if you burned it at a very high speed), you most liikely need to use some sort of "cheat codes" as you boot. Knoppix 3.4 is much worse about this than early versions (in 3.1 I needed no cheat codes, in 3.4 I need two different ones or I can't boot and hang like you do). In my case I need "knoppix noscsi acpi=off". You can try that, but you might need different codes. I know of no good way beyond trial and error to find what code or (much worse) what codes you might need. Sorry.