Recovering from Ctrl+S in Putty

The problem is related to XON/XOFF command that is mapped to Ctrl+S sequence. The terminal doesn't echo the commands you issue, so you need to remember press Ctrl+Q in order to turn flow control ON. There is a way to ignore such behaviour. What you need to do is to change your terminal characteristics.

stty -ixon

Consider add this command to your /etc/profile.d/ixon.sh file.