The first thing to do is to use jumper cables and use a second vehicle to make sure it is not a tired battery. Let the vehicle run for 10 minutes attached to the jump terminals under the hood. Then give it a try. If it still does not start, you should try another key. BMW uses a chip in the key and a bad key could be the problem.
If these two things do not work it will need to be scanned by the dealer to check the EWS system that sends the start message to the starter. The statuses can be checked on the BMW equiptment to save you diagnosis time so they can determine quickly if it is the starter, EWS, DME, or a bad key. If you do use the dealer please bring all of your keys when you have the vehicle dropped off as this will help the diagnosis process.