Car died on me. No starting at all

Hi guys, Hope you can assist.  i was still driving my car when it just simply
cut out.  I pulled to the side of the road where she died completely.  I
managed to revive her once.  But there was not enough power to keep her
idling. I had to have the car towed in.  I further then checked all the fuses.  I
replaced the battery in my immobilizer.  I checked the battery.  All the
dashboard lights come on as well as the lights and brights.  The radio
everything.  Tried towing the car to get it to start in one of the gears.  Tried
push starting the car.  Tried jump starting the car.  No joy at all.  Which
makes me doubt it is the battery or the starter or selenoid.
We tried plugging in a diagnostic tool but it does not pick up the ecu at all.
Does that mean i need a new computer box.  Is it possible it could be
something as simple as the key coding.  Is it even possible to just lose power
and cut out.  Please help me with some ideas as to what it can be.  I am a
woman and are sick and tired of being ripped off by mechanics just because
i am one and not supposed to know any better