Take a spark plug out, put the lead back on it and lay it on the engine so it earths and crank the engine over, you should see a spark jump the gap.
If no spark start checking back through the ignition system, points closed up, wire off, coil etc.
If you get spark then the fuel system will be the issue. Basically only 3 things stop an engine from starting/running, mechanical failure (rod hanging out the side etc), spark and fuel. I did say basically!
Have you checked the wiring from the switch?