I'd bet that it's the timing belt. I had that happen to me. It just quit running all of a sudden.

Check and see if the motor seems to spin over too easily since it basically has no compression. You might be able to barely see the belt on the cam pulley behind the plastic front cover. If it's broken, it won't be tight on the pulley. Also as somebody else here said, look through the oil filler while someone cranks it over. Good luck!