Need more. My first guess would be the starter but I think more info is need.

What happens when you turn the key? Is there a clicking sound?

Does it the engine turn over?

Is there dead silence when you turn the key?

All dash lights come on when you turn the key? if yes, what does the engine appear to be doing at the same time?

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2000 X5 4.4i w/ 150K miles.