I saw a question in SuperUser titled How do I restart a frozen screen in Ubuntu without losing any open windows? and I think it would be helpful, but the problem is I can't switch to any of the TTYs.
I'm trying to switch to tty 1 with Ctrl+Alt+F1 and it doesn't work, and honestly I even can't say if it has ever been worked before.
The problem already occured one time about one month ago and fixed after 3 hours of waiting but this time the difference is that the cursor is moving and the clock is working right. Moreover all system monitor indicators show current state of hardware so it seems that the processor, disc, RAM and swap are also working. It happened during moving a terminal window to another part of workspace which is displayed on second monitor. And, what is strange, I can see mini dialog box with the resolution of the window that I have never seen before.
Trying to unplug the second monitor didn't help. What can I do to unfreeze it and to not lose unsaved data?
Shortcut Super+D made GNOMEs windows unfrozen so in similar cases this should be helpful.