As you have found this page I am sure you have accidentally hit the control+s short cut whilst inside a PuTTY shell and following that no keystrokes appear to affect the session. Basically hitting ctrl+s causes PuTTY to stop executing the stream coming in from the keyboard. It does however still listen to your keystrokes and it basically adds them to a queue.

Hitting control+q will re-open the stream execution, but it is worthwhile noting that it will also execute all the queued up commands as well!