Bummer Norman... It has to be one of the big 3, gas, air or spark. Let's do the checks in the order of simplicity though;
- Air - Pull the air filter, clean and in order? Since you have it out, shine a flashlight in the hole and have a peek in there. Any critters? Is it open and clear?
- Air - Check the snorkel area, bugs have been known to make it their home.
- Gas - Open the gas tank fill cap to ensure it is not vacuum locking.
- Gas - Is the gas getting to and filling the float bowls? Loosen the drain screw to check each one.
- Gas - Take out a spark plug and see if it is wet from gas.
- Spark - Take out a plug and ground it to the engine casing. Press the start button and see if you have a nice blue spark. (repeat for the other plug)
Lastly, have some patience and keep working with it. It may take some time to fully prime up the carbs after a long stent in storage. It took my 250 30 mins to fully come alive after 4 months in storage over the winter. Nothing was wrong with it, because I tracked it the very next day.