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it sounds like you may have simply drained the battery so I would start by trying to jump start it, or charging it
if the car still shows zero signs of electrical life with jumper cables attached then you can also check battery connections and check fuses.
if still no luck, then this would most likely be due to a failed battery unit (the box on top of the battery), burning/melting of the positive battery cable or fuse box, or failure of the car access system.
these possibilities would unfortunately require professional intervention and parts replacement, so in that case it would be advisable to have the vehicle recovered by flatbed tow to your nearest BMW garage for a proper diagnostic and repair.
Sorry to have potentially bad news, please don't shoot the messenger - I can only give the honest answer, which I have. Please remember that a low rating PUNISHES me for my honest and prompt response, and I receive nothing for my time and answer until a positive rating is clicked.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your repairs.