The most common causes of this are problems with the fuel supply or the ignition system.
Use choke when you start the engine for the first time after it has not been used for a while. Remember that using the choke when the engine is warm can flood the engine. Wait a few minutes before trying to start it again if the engine has flooded.
If the fuel tank is empty or contains old/contaminated fuel, it will need to be refilled with new, clean fuel.
If neither of these work, the fuel filter may be dirty or need replacing. The spark plug may be fouled and need replacing. The plug leads may be loose, and should be firmly pushed back in place.
If the problem persists, we recommend you contact your local dealer. A list of dealers is available under Service locator above.