As I was working today, my Vim instance stopped responding completely. I ended up killing the tmux tab that Vim was in and starting a fresh one. I thought it was weird that Vim froze, but I just moved on. Then it happened again and I noticed what caused it: I accidentally pressed Ctrl+s. This causes my terminal (tmux is the culprit, I think) to pause accepting input. So it appeared that Vim had frozen, but it actually had not.
To get out of this predicament, just press
and you can resume your Vimming. Pretty odd, especially since it appears to issue all of your input-while-paused at once. I’m not sure what the purpose of the pausing is, but I’m guessing it’s useful to someone.