Engine shut off while running, cranks will not start

Okay here it goes. A couple days ago it quit running.The engine will crank and turn over but will not start up. Also the engine light will not come on and my diagnostic scanner can’t connect with the ECM. First thing I did was to check all my fuses but they were all fine. So then I pulled the ECM and check the power and ground wires and all were fine. So then tried putting a different ECM in but that didn’t change anything. so now I’m not sure what to do.