It could be a few things. You could of tipped your mower the wrong way and got oil in the air filter. Check you air cleaner and replace it if it is soaked with oil. You also may have over filled it with oil. Check your dip stick and make sure the level is correct. Drain off the extra if it is over filled. It should be one these problems. After you correct it you'll have to run the mower for a few minutes for the smoking to clear up.
What engine is it? Do you know the model number of the engine? It's usally stamped in the sheet metal right above the spark plug.