Ctrl+S happens to be a rather handy and popular combination as it’s used in Windows applications to save your current working data. I accidentally used it several times in Vim and it keeps locking the screen up and halting the interactivity — basically, after Ctrl+S I can’t do anything to the terminal (window). I use PuTTY in Windows to access my Linux hosting via SSH.
A few lookups in Linux manuals reveal the secrets, the Ctrl+S key combination temporarily freezes the screen but the input is recorded though not dealt with. Once you press Ctrl+Q, it is unlocked and all the flood of input you have provided to the terminal takes immediate effect.
So, don’t panic when you accidentally pressed Ctrl+S and seemingly freezes everything, just press Ctrl+Q to exit it.