I've had a similar problem. I'm not sure if the only workaround I've found will work for you but it's worth a try.
I switch over to tty1 (Ctrl+Alt+F1) and log in which hopefully you can do. If you can't even switch then your problem is worse than mine! But assuming you make it that far I've found that putting the computer to sleep and bringing it back up again at least makes the computer responsive again. It's really not an ideal solution. But to do this
After that just hit the power button to bring it back up. If you switch back to your graphical terminal (Ctrl+Alt+F7) things might be responsive again.
Not ideal but it's better than a full restart especially if you had important stuff not yet saved.
I haven't yet found what is causing this otherwise there could be hope for a better solution. Somebody in the thread I linked to suggested that gnome-screen-saver is causing the problems but killing it after a freeze didn't fix anything and killing it at the start of a session didn't prevent a freeze for me.