When you use terminal/putty/secureCRT/xshell,you can mistakenly type ctrl-s.It makes terminal freeze.It is because ctrl-s is an flow control signal to terminals.To resume the terminal,you should push ctrl-q.
This is not very convenient solution.We use ctrl-s a lot, especially for EMACS users.To prevent this,you can add this line in your login script.
stty stop undef
You can confirm this setting using this command.
The stop section should be undef.