An engine needs 3 things to run. Fuel, air and ignition. Most always it is getting air so we'll skip that. A quick check if it is a fuel problem is to use a starting fluid spray. If it causes a attempt at start then fuel system problems would be traced. If not, then more than likely it is ignition. First I would see if spark plugs are sparking. If so, then they may not be doing so at the correct timing. Unfortunately there are hundreds of things that can cause an engine to not start. I can only give you a couple of starting points to begin your search.