Let's see -
ECM 720b relates to missing timing signals
ECM 500b relates to a missing crank trigger signal
ECM 5000 relates to a faulty AC system pressure signal (did your AC work before this?)
So it sounds like either you have a failed crank position sensor or a fault with the ECM itself. It could also be a wiring issue but I'd say a failed crank sensor is the most likely culprit.
Unfortunately failed crank sensor means the car is a brick until you can install a new sensor. Fortunately, it's generally not that hard (although you need to tell us what year and engine type you have, or drop the VIN and someone can look up the specifics)