My neighbor had the same problem with his Crapsman Rider. What normally happens is when you move the lever to engage the blades, a pulley on top of the mower deck puts tension on the belt connected to the blades and the blades start turning. The pulley and connected components get rusty and stuck which prevents the pulley from moving when you try to engage the blades. The pulley assembly is not easy to get at with the deck installed on the mower. We had to take the mower deck off and free up the pulley and related components which were rusted. This solved the problem, but it's a poor design to say the least. I expect the problem to return eventually. If you hose off your mower with water after you use it, then that could have caused the problem.