I'm sure this happens to lots of people with a Windows background (or even people who live in 'modern' editors like Kate or gedit). Editing in vim, and instead of hitting Escape and :w to save the file, I hit Ctrl-S. Instead of saving the file, the vim session 'hangs' and won't respond to the usual keypresses. This happened to me once before and I must admit that the last time I manually killed the vim process. This time around Google was at hand, and now I know I'm not the only one who's had this problem :-)
Solution: it's not vim's fault, it's actually an xterm thing. In xterm, Ctrl-S is a kind of scroll lock and locks the console. Hitting Ctrl-Q gets things back to normal.