Today I stumbled over an frozen vi, after using ctrl-s (from a remote session). To resolve it simply use ctrl-q.

This can easily happen if you switch from emacs, where C-x C-s saves the buffer, to vi, where you have to use ESC :w.

The reason for this is the XON/XOFF protocol. Sending ctrl-s is XOFF, stopping the communication (in one direction), and ctrl-q sends XON, reenabling the data exchange.