How can I fix my Yard Machine 14.5 hp, 42" cut, riding lawn mower?

My Yard Machine Riding lawn Mower fires right up, but it doesn't seem to want to go anywhere. I'll start it up, shift to forward, and set the cruise control, and it won't move. I checked the belts underneath to see if they've slipped or something but they are on. I don't know what else to do.