Agree with Rivets; likely a dirty carb.
You can drain the gas and try a product called "Mechanic In A Bottle" to get it cleared out. This stuff is sold in a two-pack of 2 ounce bottles at Wal-Mart and other retailers for about $6-$7.
With the tank drained and clean, you put the MIB in the tank, and push the primer bulb until it is in the fuel system, then let it set for about 3-4 hours. Put a little bit of gas in the tank, and try to start it. If it starts, the product cleaned out the carb. If not, you may be looking at removing and tearing down the carb to get everything clean.
I've used it once in a 4-stroke mower with good results. Will be using a second bottle to help a friend with a non-running tractor that has all the dirty carb symptoms.
Joe