Try Ctrl-Alt-F1 to get a console. Log in as root and then run 'halt' or 'reboot' to either turn off or reboot your machine.
To get your mouse working
http://panuganty.tripod.com/debiantips/mouse.htm looks like some simple instructions.
Remember that DeMuDi
is Debian with a special kernel for music makers. So log in as root, and run 'modconf' and make sure that the modules for an USB mouse are being loaded. Then run 'dpkg-reconfigure xserver-xfree86' (or xserver-xorg if you are running xorg xfree86 for X-Windows) to setup your mouse.
Daniel