Put a spark checker on the motor - set to about 1/2 inch, make sure it is ground (clipped) to the motor and give it a pull. Should see a solid blue spark. Try with both cylinders. If you aren't getting a spark, you can pull til you are blue in the face and it will never start. If no spark- you have an electrical issue. Could be the kill swithc mentioned above, a bad ground- I have points and condensors on my 15hp- you might have something different.
If you are getting spark, then you likely have a fuel issue. You can put a clear hose between the fuel pump (diamond thing on the side of the motor) anf the carb and see if fuel is flowing to the carb. If fuel is going to the carb then the motor should at least sputter. make sure the choke is pulled out (throat closed) and the throttle is advanced to the "start" position ( I pretty much put it wide open and reduce throttle as soon as the motor turns over. I can't get my 15hp to start without opening the throotle, so that may be your issue. let us know.