The most common reason riding mowers stop moving is the belt is broken, worn and stretched, or off a pulley.
Look under the mower with the engine off and the clutch out. The belt should be tight on the engine and the transaxle pulleys. Get someone to press and release the brake/clutch pedal while you watch the idler pulley bracket. The idler bracket should move to tighten the belt when the clutch pedal is released.
If the pulley on the transaxle is spinning, the problem is internal in the transaxle.
Here are some DIY guides for Riding Lawn Mowers that Won't Move: searspartsdirect.com/diy/symptom/riding-mowers-tractor-repair/1234652/won-t-move/ltr1818
You can find parts, diagrams, and your owner's manual on Sears PartsDirect: searspartsdirect.com/model/5nig677kmi-000247/craftsman-917287011-front-engine-lawn-tractor-parts
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician by calling Sears Home Services at 1-800-4-MY-HOME.
Hope this helps!