Is it safe to assume you have tried a quick push of the
power button/any-key or swipe on the mousepad to bring the screen
back? If so, what if you try "control+alt+F3". Do you see a login
prompt? If yes there, it is likely we can get you back into your
system without having to shut down. Also, you can set up the
preferences on screen close like Ian suggested, but note that you also
need to disable the screensaver which is a separate menu option in
system -> preferences. However, on one of my laptops, the disabling
the screensaver refuses to persist across boots.

Don't forget when forcing a reboot, it is safer to hold down your
"alt" key while slowly pressing, in
sequence,"Printscreen/sysrq,R,E,I,S,U,B". You can search this list or
the web/wikipedia for more info on that series of comands if you are
not familiar with it.