2001 Cadillac Deville sudden engine or electrical failure w

My car will suddenly quit running, the dash lights go out, no power at all, while driving. I can be driving down the Interstate with my foot on the accelerator and it will cut off, or taking offfrom a dead stop, coasting, changing gears, or at any time and the car will suddinly just quit running. I have no power steering or control of the car. I can put the car in neutral and start, or stop fully and put it in park and start it. The mechanic has connected it to a computer while running and cannot get it to simulate the problem. Do you have a diagnosis?