could be alot of things , should have the dealer check the control unit faults .
sounds like it may be a key going bad .'
thanks
the key not only has the remote for the alarm but also contains a chip for whats called the ews system that will not let the vehicle start without a valid key . if this happens with all keys it may be a faulty starter or ignition switch . the switch would be the easiest and you could do this your self with a few basic hand tools . if the switch doesnt repair the problem i would recomend taking to the dealer and having them check the fault in the vehicle , there may be no faults stored though . if its the starter that is the cause i would let them do it .
thanks and let me know what you find .
BMW Technician
12 YEAR MASTER TECH