Is this a lawn tractor?
Did you tip it up on its side?
Some mowers have a breather pipe running from the crank case to the air filter. In certain circumstances tipping them can cause the oil to run up the breather into the air intake. When the mower is returned upright, the oil will run down the carb into the cylinder. It may run fine and drive into its shed, but next time it wont turn over.
To confirm, remove the spark plug and turn over the engine. If oil comes out, clean the air intake and carb with carb cleaner. If not, other fault.
Jmmy, March 2010