You should be able to logout of the root session which will take you back to the console you used to start it and then you should be able to use <CTRL><ALT><F5> to get back. It seems to work best if you hold down CTRL and ALT and then press the function key, also you may want to try to move back to a console and then to your knoppix X session.

I have been doing this on my (11-08) knoppix that I'm running on a computer in one of our chem labs at school.

If you are running knoppix live and want it to stay running for a while I always boot into runlevel 2 then if you logout or crash kde etc. the system is still running, and you can start up kde again, etc., ie. you won't halt the system.

rock