Vim is Stuck!
This post lists Control-q which may un-stuck Vim. It also lists some history about the problem.
Try Control + q
If this worked, you pressed Control + s in a terminal emulator like GNOME Terminal.
In a terminal Control + s stops terminal output. Control + q restarts it.
Steve Oualline lists this on p156 of Vi IMproved-Vim ISBN: 0-7357-1001-5 available from Amazon at link.
The VAXstation 100 pic is a screen capture from link