My Toro riding mower is sitting in my yard with the rear wheels locked. The motor will start, the cutting blades will rotate, but the riding mower will not budge, and it cannot be pushed. The rear wheels are frozen! What is wrong?

Hi,
Is it a hydro? How about a model number. I could say that the brake is stuck in the "on" position. That would do it, but many other things might also. What was happening when it ceased to move?
Peace,
Clarke

Is the parking brake engaged if it has one?