This could be far from your actual issue, but in my e64 m6 the same thing happened to me. The car would crank but it would never turn over. The dealer said some module was not able to validate my key and as a result cut the fuel. To fix the issue they reset the ecu (something along these lines...).
This happened to me in my 07 e63 M6 and it turned out that the CAS module was not synced with the DWA alarm module so I synced them and the car started so I think it's either that or a fuel pump issue, whether electrical or mechanical, can be as simple as a relay as mentioned above.