I was mowing my lawn when the mower blades shut off. The mower was still running so I drove it to the barn. I turned it off to look at the belts to see if they were good and they were. I went to turn it back on and nothing will happen. I have replaced the pto switch and that did not help it still will not even turn over. It is a craftsman dyt 4000 riding mower about 4 years old. Does anyone know what the problem could be.