Fuel, spark, compression.
Start by spraying easy start in the air filter - see if it fires. - if it does then it a fuel problem if it doesnt its either a spark problem or a major engine fault.
If it doesnt fire then change the plug - sometimes a plug will spark when under no compression - ie outside the engine but not when installed.
If none of these work stick your finger over the spark plug hole and turn the engine over - it should puff and not suck.
If you find you have a fuel problem sometimes blocking the air inlet with your hand and turning the engine over with the throttle fully on 'hare' will pull through a blockage. Sometimes tapping the carb with a spanner can drop any stuck floats.
Best of luck - damned machines never work when you want them!!