BMW 328is coupe will not start

What is your question?

I tried to start my 1996 328is. It will not turn over. Their is no noise at all, no lights, seats, doorlocks, etc...I haven't driven the car in 3 weeks but it has been in a coverered garage. I didn't leave any lights on and the battery and alternator are only 2 years old. Obviously my battery is completely dead but does that mean my alternator is too? What could have caused the battery to die? Or is it something else? Any advice is greatly appreciated.