You should try to search first, there are tons of posts about that.
Self level Susp. Inactive means that your rear AIR suspention is faulty and is not riding normally. There can be few problems there
1. Does it rise up and then still drop? Do you hear any sounds when you unlock the car? (from the wheel, sound that air is coming out) that means Air Spring has leaks.
2. When unlocking car, Listen in the trunk if the Compressor works.
3. There are sensors to identify level of ride, they can be faulty.
4. Air lines from compressor to Air spring might be leaking causing car to fall down, However It has to rise up unless lines are un-attached.
From my experience Air Springs are likely to be faulty.


Here is Picture.
#1 Pneumatic Air Spring
#8 Air Supply Device - Compressor
#18 Sensor

Hope that helps.
EDIT: In terms of Sensor, Car usually goes up, so sensor is least likely to be the problem in this case.