Common problem among Bimmers. They will have to connect jumper cables underneath the car with the positive on the start and the other grounded and connected to a car that is not turned on. This should provide enough juice to open the doors. In this case it is probably your battery which is commonly locate under the rear passenger seat and a pain to get to. Replace the battery and run the car for several hours. Most often the cause is leaving the lights on. If this continues and the battery tests well then probably alternator.
If you have AAA membership they may come to the rescue, if not a local locksmiths may be able to open the car for you or your BMW dealer may be able to cut a key for you i you give them the VIN number.