If you find that pressing Ctrl-Q and Ctrl-S has no effect at all in a specific GNU screen terminal than you most likely have accidentally switched on XON/XOFF flow control by hitting Ctrl-A F. The symptom is that you cannot save using Ctrl-X Ctrl-S in Emacs, and you cannot use Ctrl-Q for quoted-insert in Emacs. The Ctrl-S and Ctrl-Q keys are silently ignored.
To make the Ctrl-S and Ctrl-Q keys work again you must disable flow control in screen: Press Ctrl-A F until "-flow" is displayed. Now the behavior should be back to normal: GNU screen no longer handles Ctrl-Q and Ctrl-S specially and passes these to the application.
