It might be the starter Shane, but it also might be a deteriorated battery or wiring. I don't know if the GV400 has a decompressor, but if it does, and it isn't working, that could also cause the starter to be unable to turn it over properly. (This is an endemic problem with the B&S OHV engines, on which the effectiveness of the decompressor depends critically on the tappet clearance being exactly right. Many, many starter motors have been destroyed by people who had not adjusted their tappets properly.)
At the moment I don't see a potential explanation of why your engine stopped while you were mowing. It would be unusual for two unrelated faults to occur simultaneously.
