We were mowing and our john deere 345 just quit. Will turn over and can smell gas but will not start. What could cause this?

Hi,
Depnding on the age of the machine, it could be the camshaft. Older machines had a composite cam and the rivets holding the gear to the shaft eventually give out. Newer engines it could be the module tied to the left rear top corner of the engine. Does not really look like a module, but has wires coming from it. That machine has a fairly complicated electrical system and first rate dealers will have a test kit and analyzer to check the rest of it.
Peace,
Clarke