I've recently had the same problem with an E34 525TDS that stood
without its battery installed for a while. I am told by a BMW tech on
a paid on-line service that the BMW DWA system is responsible.
Apparently the chip codes in the keys and the car can get out of sync,
and the DWA system kills power to the starter and the fuel, so even a
pull-start won't work. In your case, you could try every key you have
for the car, it might work for you. There is also a 'urban myth' that
I can't attest to - if you turn the car on and off 220 times the
system is said to reset - you'll just have to try it. Other than that,
the car and the keys have to go to a dealer I'm told, unless someone
here knows another solution.

JC