This is from the bottom of the page of the knoppix cheatcodes:

" Caution: X-Screensaver: Don't start xlock or any screensaver that
requires a password. There are no default passwords on KNOPPIX,
i.e. all accounts are LOCKED unless you explicitly set a password.
See also README_Security.txt about this issue.
If you accidentially hit the screensaver button in KDE,
switch to one of the textconsoles by Control-Alt-F1 and kill
the screensaver (or just set a password for the knoppix user).

If you would like to edit your X-Server configuration manually
(/etc/X11/XF86Config-4 for XFree86 V4.x), use "knoppix 2" to boot
into runlevel 2 (textmode only) and, after changing the X
configuration, start the X environment with "init 5". Note that
you can always leave the graphical environment with "init 2", and
restart it later with "init 5"."

I included the 2nd paragraph because you may need to redo your xserver.
Perhaps somehow you set a lock on the desktop/screensaver and that's why the mouse doesn't
bring back your desktop. It's my best guess. Read that 2nd paragraph to restart x or do "startx"
Good luck.