Assuming you have ensured your fuel selector is on or on reserve, and your safety tether is attached (not to insult, but I've seen it done this year)
Do the obvious checks first before assuming something is wrong.
1. Check for spark - If you arent' confident in your ability to determine spark intensity based on sight, buy, rent, or borrow a spark tester to make sure you have adequate amount.
2. Check for fuel - If your spark is good, pull the plugs to see if they are wet after cranking. You could also pull the feed hose off the carbs and crank to see if fuel is being pumped. If you aren't sure if your engine is getting adequate fuel, pull the plugs and pour a little gas in the cylinders. Replace plugs quickly and crank. If she starts, then you know you aren't getting fuel because the fuel in the cylinders allowed the engine to start (assuming you put enough in). If she runs for a short time and then dies, you burned off the fuel you poured in and still aren't getting enough to keep her running. If she still doesn't start and you are confident you have both spark and fuel eliminated, then your issue is a little more serious.