The trouble with faults like this is that they are hard to trace when they only happen once in a while.
I had a similar issue with my 3.0d - it turned out to be the fuel pressure rail sensor. Having gone faulty it was incorrectly reporting to the engine management system that the fuel pressure was low - at which point it wouldn't open the injectors and therefore not start.
It will be worth getting the fault codes read.

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'02 X5 3.0d, Auto, Topaz Blue