The Blade Brake needs to be adjusted. It is braking the blade and the engine.

After again verifying that mower drive belt is routed inside all belt guides and engaging all pulleys (see manual), make sure that Deck clutch rod and lever are assembled together with spring retainer in place. Start engine, engage mower. Deck blades should begin operating. If not, re-check all pulleys to turn freely with no slop. Also - with engine off - engage mower and check to see that belt is tensioned. If not, belt stretched or mower clutch mechanism faulty or needs lubrication. If needed, see link below for downloadable manual. Here is a link that might be useful: manual

One end of spring is connected to a stationary bar and other end is connected to belt pulley that the backside of belt rides on and is tensioned by the deck release hook if it is a 48" or larger deck. This pully looks like a double pulley and is flatter looking. Look to be sure the spring has a hooked area at both ends. If not then one end is broken off and belt can't be tentioned and you need new spring. Tim

Some Model, Type, Serial numbers would be helpful! Is this handle the one that rides up in a slot and moves to the right into a hook-shaped slot? Some times they get rusty, and take a determined pull to get it to move. does your machine set out in the rain and etc.? Do you mow in dusty conditions? Have you run over any sticks, etc. lately. Have you cleaned out the clippings on top of the mowing deck. there might be a stick stuck under there, which is keeping the mechanism from doing what it has to do when you try to move the handle! All it takes is to get a stick stuck between the belt and a pulley, or in the works somewhere. Lay down on a dry piece of cardboard, and using a bright flash-light, peer under the tractor and above the deck, for anything untoward! HTH: Rusty Jones