I was mowing along just fine and then the blades of my Simplicity 4212 hydrostatic riding lawn mower just stopped turning. Belt is in tact. There is about 4" of extra play in the belt that drives the blades. Can't spot where a pulley would have come off and can't find one laying around. There was no loud noise or anything, just a change in the sound of the blades slowing and stopping. Now they won't engage at all. Any and all suggestions would be appreciated. Thanks!

Ironclad here! I'm no expert, but I did a lot of mowing in my time! Have you checked any and all fluids to the mower?