Add the this to your xorg.conf, in the “ServerFlags” section:
Option "DontVTSwitch" "true"
From “man xorg.conf”:
Option "DontVTSwitch" "boolean"
This disallows the use of the Ctrl+Alt+Fn sequence (where Fn
refers to one of the numbered function keys). That sequence is
normally used to switch to another "virtual terminal" on operat-
ing systems that have this feature. When this option is
enabled, that key sequence has no special meaning and is passed
to clients. Default: off.
As you probably don’t have an xorg.conf, better create a file in /etc/X11/xorg.conf.d/ named 10-myserverflags.conf (the exact name doesn’t matter, but it should end in .conf) with this content:
As gogalthorp already indicated, you didn’t “crash” your system. You just switched to console#12 (also called “virtual terminal #12” or “VT12”). You can switch back to the graphical system (which normally runs on console#7) with Ctrl+Alt+F7, or to a text mode console via Ctrl+Alt+F1 to Ctrl+Alt+F6.