No start can be frustrating, but keep one thing in mind...if an engine has sufficient compression, approximately correct fuel/air ratio, and spark at approx the correct time, by the laws of physics, IT HAS TO RUN. If it ran previously and won't now, it had approx the correct mixture which has NOT changed. Don't start changing the idle screws until its running again.
Start with the simple things first.
1. Drain the float bowls of the carbs to ensure there is no water in them. An easy way to see is to put the drained fuel into a glass jar, the water will drop to the bottom and you will easily see it.
2. Make sure the battery is in good condition. Sometimes a weak battery will not have enough current to turn over the engine AND fire the plugs when they are under compression.