Murray 40507X8A won't start!?

I got a Murray 40507X8A used from my neighbor today- he bought a new battery, gassed it up and drove it over. I parked it for about two hours and then started it up to mow. It started and ran no problem. I got most of the lawn done and ran out of gas. Upon refilling the tank, I turned the key and got nothing. No click, no turn over of the engine, nothing. Is there a kill switch I have to reset? Where is it? Or do I just need to wait until it resets itself? How long?

Thanks for your help!

Michael, michaelcnh@yahoo.com