The things that come to my mind are, bad main realy, bad DME, AFM has been messed with and is not adjusted properly, dead AFM, not seeing a crank/ignition signal, bad engine temp sensor (different than the sensor that sends the signal to your gauge in the cluster), very little or no gas in the tank.
When did the not starting problem first start? Did you change some parts or mess with anything right before this problem happened? Was the car sitting for a long time? Did the car run rough then not want to restart?
The main thing 90% of people forget when trying to work on their own car is start simple and don't just throw parts at it. If most people would add up the $$$ they spent on parts that didn't fix the issue before they took it into our shop it would more than cover the diag time we spent figuring out their BMW. I see it all the time at the shop I work at. Its usually some guy that is too proud to have a shop work on his wife's/GF's car that ends up bring it in for us to fix cause the hundreds of dollars in parts he threw at it didn't fix it. And very rarely does the parts they replaced help the problem get better.