I would have a professional equipment shop take a look at it.
You say you changed the spark plug, are you SURE that its the right one?
I'm not a mechanic, but I do know that lawnmower, weedeater, chainsaw engines and the like, are not complex at all. There are at least one of two things missing if it isn't running.
spark and/or fuel.
If it dies, pull your spark plug immediately. If its wet with gasoline, that means either you've got no spark or not enough spark for some reason, OR too much fuel. If this is the case, and you try to start it again, you're not going to get it to start because the cylinder is flooded, and the plug is too wet to spark.
If its not wet at all, that means you're not getting enough fuel. I can't tell you how to fix it, but these should help diagnose it, and allow you to research further.