100% ditto with ccfj1's diagnosis.

Based on the year of your car, you probably have the MKIII navigation computer. Those are vulnerable to power loss. The most common cause of MKIII failure is any work on the electrical system, or a drained battery. You are in the market for a new nav computer. I would upgrade to a MKIV, as those units are bullet proof, and the extra features are sweet.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input