Does it sounds like it's trying to start? Could it be flooded? If so, pull it over with the throttle wide open.
As was said before, the fuel could have been contaminated with water(?) You can pull off a fuel line and check it, it's very easy to tell water from fuel.
When I have fueling problems, I like to put a small amount of fuel down the plug holes. If it fires up for a second, you know within a reasonable amount of certainty that you have a fuel problem.