Check the level of the oil in the engine. It sounds like either the engine oil was overfilled or gas has leaked into the engine block.
Pull the dipstick out and smell the liquid on it. If it smells like gas then we have some work to do. If it doesn't smell like gas then we are just overfilled with oil.
Let's assume that we have a gas smell on the dipstick. When you'll need to do is go to an auto parts retailer and get some carb/choke cleaner. It will be in a spray can. You will need to clean the bowl and float on your carburetor. When you take the bowl off the carb you will see a little plastic float (kind of looks like a donut). On that float there is a little needle that opens and closes off a jet. Spray that area real good with the carb/choke cleaner. Try to spray up in the little jet hole. Clean the bowl out real good with the carb/choke cleaner as well. Put the bowl back on the carb when you're satisfied with the cleaning. Then take the spark plug out of the engine. You will need to clean it up or replace it (I replace them when this happens). Next drain all the engine oil out of the engine. Once you have the oil drained, with the spark plug out, tip the whole mower forward. You want the spark plug hole facing the ground. You may (or may not) see some oil and/or gas come from the hole. Let it drain. When it quits draining set the machine back on all 4 wheels. Fill the engine with fresh oil. Make sure you have gas in the tank. Put the spark back in and try to start the mower.
It may be hard to start at first; in fact you may not want to choke or prime it at first. When the mower starts it will smoke a lot. Let it run for about 10-20 minutes and the smoke should quit and things should be back to normal.