This could be the 'black screen of death' that can befall users of Knoppix 6.2.1 or Knoppix 6.3 when running with an Intel graphics chipset.

As you do not say what your graphics chipset is only you can confirm this.

I had this problem with my laptop and solved it by giving the following cheatcode when Knoppix is booted (when there is a small delay booting from the DVD you can type say:
knoppix i915.modeset=1

This will start knoppix with the cheatcode i915.modeset=1

This only works with intel graphic chipsets so if you have something else you have a different problem.