The problem is that the DSP amp is not communicating with other components it needs to see on the car's I-Bus. If your nav is working normally and the radio functions are normal (other than the shut-off) then you have a mystery. Does the volume control work, either on the dash or steering wheel?

The other potential cause is a failing battery. How old is your battery? I have seen this symptom first-hand when my battery neared its end of life.

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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