Take the air filter off and try it again. If it starts then your air filter is clogged. Re place it. If it still does not start, there could be water in the gas. Drain the gas, remove the gas bowl with the needle float in it, be careful not to upset the needle, remove the gas from the bowl. If there is water in the gas, you should see it there. Put everything back together and fill with fresh gas, then try to start the mower again. Make sure you are using the choke when trying to start it and make sure the blad is not in the engaged position. This is a safety feature, the mower will not start if the blade is engaged. Some seats have a saftey switch too that prevent the mower from starting if you are not in the seat with the clutch pressed or the mower in neutral.