Just happened to me last Sunday (stuck in neutral). And it wouldn't start.
Ended up be a faulty SMG pump for which there is a SIB posted in another thread (search for smg pump failure). The tech said last year when they got a few in BMW had them replace the whole transmission... but now they have found that it is the result of a faulty pump that doesn't allow the SMG to pressurize. total of 4 days including overnighting the pump in... up and running fine. It actaully feel stonger than before.
here is the link
Recall for SMG electric motor