Pressing CTRL+S in PuTTY disables the output to screen. It will appear as if your connection is dead, since you won't see anything happening when you type commands. The fix is to press CTRL+Q to enable output again.
No need to open a new session to the server. Whatever you've typed while output was off (after CTRL+S) will be visible and you can keep working.
CTRL+S sends XOFF
CTRL+Q sends XON