You need to start by isolating your problem. It can be one of 3 things. No air, no fuel
, or no spark. Fuel
problems are the most common with spark being in second, air is very rare to be the issue.
One easy thing to do to point us in a direction quickly would be to duplicate the condition and when it wont start spray a small amount of starting fluid down the intake and try starting it again. If it fires right up it is most likely a fuel delivery
problem. If it doesn't fire then it is most likely a spark problem and we can guide you from there.
Just to be safe though and cover all bases make sure your choke isn't engaged partially.