If you use Emacs a lot, you get used to typing CTRL-X CTRL-S to save the current file. But if you're trying to switch to vi, you'll no doubt find yourself very frustrated if you type CTRL-X CTRL-S out of habit, because it seems to freeze up the editor.
In vi, CTRL-S is used freeze the output. It is meant to let you pause a quickly scrolling display.
To fix the problem, press CTRL-Q. It unfreezes the display.
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