Rear ended and dead

[oracsol]

While my wife was waiting for the light to turn green another driver decided to try and defy one of the laws of physics (no 2 pieces of matter may occupy the same space at the same time) and slammed into the back of her BMW X5 (2003). She calls me because while the damage is limited to the rear bumper and rear hatch, the X5 won't start and the airbag light remains on. Incidently she recalls that the airbag attempted to deploy because there was a puff of powered coming from the dash but it didn't deploy. Fortunately the guy has insurance but I'm puzzled as to why it wouldn't start? Everything else in the BMW was working. However the only noise I heard while attempting to start it was a single "click" noise. If there was a fuel cut off I would think the engine would turn over unless that is a safety feature as well (ie. fuel cut off switch activated and in turn no power to the starter).