It's a situation that has happened quite often to me: after I press (with a different intention) Ctrl-S in a terminal, the interaction (input or output) with it is frozen. It's probably a kind of "scroll lock" or whatever.
How do I unfreeze the terminal after this?
(This time, I have been working with apt-shell inside a bash inside urxvt–not sure which of them is responsible for the special handling of Ctrl-S: I was searching the history of commands backwards with C-r, as usual for readline, but then I wanted to go "back" forwards through the history with the usual–at least in Emacs–C-s (1, 2, 3), but that caused the terminal to freeze. Well, scrolling/paging to view past things still works in the terminal, but no interaction with the processes run there.)
Best Answer
Ctrl-Q
To disable this altogether, stick
stty -ixonin a startup script. To allow any key to get things flowing again, usestty ixany.ps: It's neither the terminal nor the shell that does this, but the OS's terminal driver.