Breaking out to the terminal

CTRL+ALT+F1 should give you a terminal where yon can log in to diagnose the problem. You can press CTRL+ALT+F7 (sometimes F8) to return to the GUI.

There are additional terminals in CTRL+ALT+F2, F3, etc...

I also find very useful to map CTRL+ESC to the system monitor, where I can kill a process quickly.

As SW01 said, you can also type ALT+F2 to execute one command. This will only work if gnome-panel is enabled (it works by default).

And fred.bear added than CTRL+ALT+T will open a new window with the default terminal.

The default keymapping for the default X terminal is Ctrl+Alt+T

Javier's answer is imo the best, but I figured more info is never a bad thing.
You can also type alt+f2 to open a small prompt. From there you can open bash or some other desired program.