When a riding lawn mower squeaks and stops when you put the blade down what is wrong?

Sounds like the pulley that spins the blade is stuck. When the blade is lowered, the belt tightens on the pulley and spins it. If the pulley is stuck, it will not spin and the belt will "squeal" as it tries to spin it. The extra load that is added to the engine when the belt is trying to spin the stuck pulley is a bigger load than the engine can handle, so the engine stalls (stops). Check the blade pulley - or it is possible there may be some debris under the deck that is preventing the blades from turning.