Broken timing belt?
Running too rich and flooding?
Pull the air cleaner. Mine will do this when it gets rainy outside. If the filter's clogged it'll sometimes cause that. Leave the airbox open a bot and try starting it. Do NOT drive it with the box open. Then HOLD DOWN the accelerator and crank, crank, crank for about a full minute to see if it catches. Don't crank much longer than that as you risk burning up the starter.
But, as someone else replies, what about the timing belt? Does it crank with the same amount of effort as before or does it seem to be cranking 'a lot' faster than before? If the belt's broken then it won't be compressing the cylinders and it'll generally crank a lot easier (sorta like pedalling a bike when the chain's gone off the rings...)
It could also be the mass airflow sensor.
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