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Make sure you didn't bump your switch that selects whether to adjust the left or right mirror. If neither mirror is selected it turns off the mirrors rotating down when in reverse.
The switch will be on the armrest of the driver side door in front of the up/down window buttons. There should be a directional button, auto, L & R. If neither L or R is selected then the side view mirrors down angle down when reversing. Make sure you have one of them selected to switch it back on.