walker blades not engaging but engine runs

I was cutting the grass today, and the blades on my walker model mc just quit. The engine did not quit. The belt seems OK, and I can spin the blower fan. It does not seem locked up. When I start the engine, and pull the switch up to engage the blades, nothing happens. Where should I start to solve this problem? Thanks much.

Have you checked the shear pin ? It holds the drive shaft coming off the engine to the blade gear box and is designed to (shear away if you engage with too many rpms . Raise the deck and try spinning the blades by hand if you can spin them freely the shear pin is gone. If they won't spin then you got other problems. 90% of the time it's the shear pin. 5 bucks at the part house.