As you say, these are very simple. The connection from the battery + to one of the bulb sockets is a single metal piece, so there isn't likely a failure there. The - end of the battery goes through the body before it reaches the other pin socket and provides the switching. This is most likely where the failure is occuring. If you lift up and remove the black plastic part that the bulb goes into (it's the one that says "Do Not Remove"), you can seperate it from the actual bulb socket/switch (if the batteries are out, you can slide the socket all the way out the end of the flashlight). You wil see a small metal tab that is part of the second pin socket and goes to the edge of the bulb holder and makes the connection to the body as it rises up to complete the circuit and turn on the light.
One cause that I can see happening during normal use is that there may be something caught in this area that is preventing the socket from rising up far enough for the metal tab to contact the body. I would try pulling it completely apart, give it a good cleaning, and reassemble and it will hopefully start working again.
Another possibility is that the little metal tab is pushed so flat from turning on and off that it no longer makes contact. You could bend up the tab very slightly to get better contact with the body. But this usually results in an intermittent light as you shake it, not a total failure like you describe.
