Well you only need fuel, air, spark and time to make it run. So since it has run, time should be good. Air, should be good since you unhooked everything. That leaves spark and fuel. Check spark at the plug (or try some new plugs - champions aren't so). Also after some cranking pull the plugs and see if they are wet. If they aren't, then I would say you aren't getting fuel. Pull the plug on the bottom of the carb to make sure you have fuel and not just a little tiny bit. You can always try pull starting it, but that shouldn't be necessary. Have you checked to make sure a valve didn't get stuck open and create no suction?
If you still can't get it to run, drop it in a box and send it COD to me. I will be glad to take it off your hands. :)