I had an issue on my 450 where it wouldn't start recently. Engine would crank, but no ignition. Checked the spark plug and it was making spark, but when I took it to my dealer, they changed the plug and it fired right up with the new one. Problem solved. Apparently, there is an issue with plugs on these machines being fouled, and even though they still spark, it might not be right when screwed into the engine. So, before you go too far down the road, I'd try a $5 spark plug (should be able to get one at any local auto parts store) and see if that doesn't solve your problem. Especially if you have not changed your plug, ever, or in a long time, this or battery level are the most likely culprits.