21 HP 46" riding mower (model 917.28852) - blades stopped turning while mowing a flat easy section. I checked the belt, it is not broken, but seems really slack. When I move the clutch lever there is no resistance, and nothing happens. Please help..

Hi,
I believe you must have left the last idgit off that model number. Should be a "0" or "2" from what Sears produced in the way of breakdowns. First I suggest that you check the cable to see if it has broken at one end or the other, or if something has wedged the idler arm that engages the deck. Put a picture of a page out of the owners manual for reference.
Peace,
Clarke