Check the PTO lever has not become engaged, perhaps accidentally. This is a common cause of starting problems - it is all to easy to snag the lever with your jacket as you dismount. With the key turned to the start position, jiggle the PTO on/off lever with your other hand. If this is the problem you will hear the engine turn over as the lever hits the correct position.
Check other safety switches have not engaged, for example beneath the seat.
Engine stalls when facing downhill, going over bumps or if you have a low body weight.
Many people, especially lighter folk, have found that the engine dies when they lean forward in the seat. This is more prone to happen when facing down a hill, or trying to observe FEL activity. One owner at least found that a 5-minute adjustment to the switch beneath the seat solved the problem. On some models the seat switch spring consists of two brass stampings. Removing just one of them will reduce the tendency of the seat to rise up, but do not remove both or you will defeat the safety mechanism completely. Adding some weight to the seat to take account of a light driver might also work - for example the BXpanded toolbag that attaches to the back of the seat. As a last resort, eat more!