Every once in awhile, I'll press CTRL+S by accident while I'm inside a terminal window. For the longest time, this simple accidental keystroke meant I had reconnect to my Linux server, kill whatever program I was running, and then start it again. Eventually I got sick of this happening and decided to do what I should have done in the first place: Google It.
Apparently CTRL+S actually does XOFF, which means the terminal will accept key strokes but won't show the output of anything. It will appear as if your terminal is dead when it's really just waiting to be turned back on. The fix? Simply press CTRL+Q to turn flow-control on (XON). If you pressed a whole bunch of keys before pressing CTRL+Q, you'll see the output from those keystrokes.
In the Windows world, CTRL+S is used as the Save command. Over the years, I've developed the habit of pressing CTRL+S every few minutes while working on a document, simply because I've had too much work lost from stupid errors. Thankfully, this habit will no longer get in my way of working in the Linux world.
Update: A commenter suggested a way to disable CTRL+S from sending XOFF altogether:
add this to your
man sttyfor more options):
stty ixoff -ixon
Update: A commenter provided a tip for making CTRL+S actually save the file in Putty'd VIM.