i was riding around in my woods today and my 400 was running just fine, then i came home to eat and went back out to ride it and i was heading back out to the woods and it started cutting out and back firing on me...i shut if off for a minute and tried to start it up but it wouldnt fire up....what could be the problem here...thanks

 

Try a new spark plug. The 400ex seems to like a new plug, sooner than any other 4 stroke I have ridden.