It's a situation that has happened quite often to me: after I press (with a different intention) <kbd>Ctrl-S</kbd> 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 <kbd>Ctrl-S</kbd>: I was searching the history of commands backwards with <kbd>C-r</kbd>, as usual for readline, but then I wanted to go "back" forwards through the history with the usual--at least in Emacs--<kbd>C-s</kbd> ([1](https://unix.stackexchange.com/questions/541/best-way-to-search-my-shells-history/2120#2120), [2](https://unix.stackexchange.com/questions/4079/put-history-command-onto-command-line-without-executing-it/4086#4086), [3](https://unix.stackexchange.com/questions/7061/how-to-search-the-whole-of-bash-history-without-needing-to-go-forwards-and-backwa/7064#7064)), 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.)