Air, spark and fuel - the three most important things an engine needs to fire. If you're confident your air filter's not blocked and you've got a spark, then you need to investigate fuelling. Start with the basics - has it got fuel? The gauge might say you've got half-a-tank but the sender could be broken. Are the spark plugs wet? If not, then you're not getting fuel. You'll need to work backward to check for blocked lines, fuel filter or a failed pump. If you're not sure how to do this, either have a look in the Haynes manual or take it to a decent garage (see our Good Garage Pages for some suggestions).