I have a 12.5ap 40 inch murray riding mower I think the model number is 289707.. and the belt came off but when I put it back on there is slack in it and it won't engage.. what do I need to look for to do.. thanks

Which belt, you have at least 3. Most belts have a spring loaded tensioner pulley that engages the belt and maintainens tension, did you pull the pulley back and get the belt around it?