lawn mower will not start?

I can not get my craftsman mower to start. It only backfires while attempting to start it. It has a 4.75 HP Briggs and Stratton vertical shaft engine. It is only a little over a year old and has always started on the first pull. I suspect it may be related to hitting a tree root that caused the engine to stop abruptly. I assumed it was out of fuel when I could not get it restarted. I had no fuel on hand so put the mower away. A few days later I filled it with fuel and now will only backfire while trying to start it.