my husband took the blades off our riding lawn mower. (Bolen MTD 38 inch ?

My husband took the blades off our riding lawn mower (Bolen MTD 38 inch cut), when he put the new blades on, the blades will not turn. I think he left a piece off because when putting the blades back on he found something that looks like a long thick screw with screws and he is not sure if it goes on the lawn mower or not. Please help my grass is really tall.