A few times I have lost the ability to input anything in a window of a screen session. I noticed this time it was after hitting CTRL+S. So a quick google resulted in....
When your screen session has started intercepting Ctrl-s (C-s in emacs speak) it is because flow control has been switched on. To switch this back off just do C-a f a few times times until -flow appears at the bottom of your screen session. Problem goes away until next time a rogue keypress switches it back on again. Phew.
I hope that helps someone.