A sharp pencil always works well! I always have one in my kneeboard just in case.
'Overload' can happen to anybody, in fact one secret of good instructing, is knowing when your student reaches this level and then keeping them just below it.
When we start flying, this level can be reached by the smallest of things. The students body is being assaulted by a magnitude of new feelings and experiences and the brain needs time to assimilate these, even a simple task (to us) such as using the radio or using the rudder/anti-torque pedals can provoke a complete shutdown. It is rare that someone becomes totally incommunicado, but it does happen.
I usually like to keep a little bit of chat going, that way I can monitor the students responses, often when you ask someone to describe what they are doing and they go completely silent, you know they are approaching meltdown, help them by removing a task, but gradually reintroduce it, keeping them calm as you do.
Never let any student get depressed/demotivated after an episode like this. It has happened to all of us and is in no way a failing of theirs or yours. The next time they go up, it will be a huge amount better, since the brain has had time to process all of the new information.
In reference to this situation Whirly, touch is the best way of 'snapping somebody out of it'. It doesn't have to be a punch in the face (that is a last resort), simply putting your hand on their arm whilst talking gently seems to work most of the time. But if you have to then Genghis' ninja style technique may be appropriate!