I had a very similar problem, but this was with a diesel so may not be exactly the same.
All the car would do was crank but not fire.
Having spent a week at the dealer, it turned out to be the fuel rail pressure sensor. It was giving incorrect readings to the management system and it just shut the engine down.
I assume the petrol engines have a similar set up, so it may be worth having a look at that to.

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'02 X5 3.0d, Auto, Topaz Blue