Air suspension light came on I turned the car off light went off, then I noticed the rear end of my car was low. What could that be?

A technician or mechanic can check this, but the most common problems are:
1) An electrical failure of some part, for example the switch or the relay that activates the compressor, or
2) the compressor itself could have failed.
God bless your efforts.

Having it hooked up to a factory scan tool would be your best bet . Check for DTC'S diagnostic trouble codes . Without testing ,code scan ,we'd only be guessing .
Very complex , the untrained person hasn't got a pray of fixing this . You have no idea what your dealing with . Vehicle Dynamic Suspension The vehicle dynamic suspension consists of the following components:

• Rear air suspension control (RAS) module (5A919)
• Snorkel
• Drier
• Air compressor (5319)
• Air suspension switch (5K761)
• Solenoid valve (5311)
• Air spring (5560)
• Air suspension height sensor (5359)
• Air line
• Rear Air Suspension Control Module
  A microprocessor controls the air suspension system. The microprocessor and its supporting hardware are contained in the rear air suspension control module. The rear air suspension control module responds to signals from various sensors in the vehicle to maintain the programmed ride height while the vehicle is either moving or stopped. The rear air suspension control module accomplishes this by opening and closing solenoid valves to control the amount of air in the air spring(s). The rear air suspension control module turns on the compressor by applying voltage through the compressor relay to inflate the air spring(s) and raise the vehicle. The rear air suspension control module opens the vent solenoid to lower the vehicle by releasing air from the air spring(s) in response to signal inputs from the air suspension height sensor(s).
• Rear Air Suspension Control Module Diagnostic Trouble Code (DTC) Index DTC Description Source Action B1317 Battery Voltage High Rear Air Suspension Control Module GO to Pinpoint Test C . B1318 Battery Voltage Low Rear Air Suspension Control Module GO to Pinpoint Test C . B1342 ECU Is Defective Rear Air Suspension Control Module INSTALL a new rear air suspension control module. REFER to Module-Air Suspension Control . TEST the system for normal operation. C1441 Steering Sensor Channel A Circuit Failure Rear Air Suspension Control Module REFER to Section 211-00 . C1442 Steering Sensor Channel B Circuit Failure Rear Air Suspension Control Module REFER to Section 211-00 . C1722 Air Suspension Height Sensor Power Circuit Short to Power Rear Air Suspension Control Module GO to Pinpoint Test D . C1723 Air Suspension Height Sensor Power Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test E . C1760 Air Suspension Rear Height Sensor High Signal Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test F . C1763 Air Suspension Rear Height Sensor High Signal Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test G . C1765 Air Suspension Rear Height Sensor Low Signal Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test F . C1768 Air Suspension Rear Height Sensor Low Signal Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test H . C1770 Air Suspension Vent Solenoid Output Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test I . C1773 Air Suspension Vent Solenoid Output Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test J . C1790 Air Suspension LR Air Spring Solenoid Output Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test K . C1793 Air Suspension LR Air Spring Solenoid Output Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test L . C1795 Air Suspension RR Air Spring Solenoid Output Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test M . C1798 Air Suspension RR Air Spring Solenoid Output Circuit Short to Ground Rear Air Suspension Control Module GO to Pinpoint Test N . C1813 Air Suspension Vent Request Exceeded Max Timing Rear Air Suspension Control Module GO to Pinpoint Test O . C1818 Air Suspension Air Compressor Request Exceeded Max Timing Rear Air Suspension Control Module GO to Pinpoint Test P . C1830 Air Compressor Relay Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test Q . C1832 Air Compressor Relay Circuit Short to Power Rear Air Suspension Control Module GO to Pinpoint Test R . C1840 Air Suspension Switch Circuit Failure Rear Air Suspension Control Module GO to Pinpoint Test S . C1842 Air Suspension Switch Circuit Short to Power Rear Air Suspension Control Module GO to Pinpoint Test D . C1897 Steering VAPS Circuit Loop Failure Rear Air Suspension Control Module REFER to Section 211-00 . U1041 SPC Invalid or Missing Data for Vehicle Speed

Check the air lines for leaks especially around the right rear tire area. chances are the flat tire severed or pulled loose one of the lines from compressor

there is a adjustment on the link between the axle and the sensor

you might have a leek in the system
check the air hose lines

do you hear the pump running from under the back of the car? check fuses? is the suspension light on in the dash? this system is diagnosable with the right tester.have someone get the fault code from the suspension module. could be as easy as a sensor. was the car towed recently? flat beds ofter knock off the sensors when car gets tied down. my air suspension went out on me. And the dealers price was too expensive and i went to Strutmasters.com and bought a conversion kit that eliminated the air and has a lifetime warranty so now i don't have to worry about it ever again. Prior to jacking or lifting a vehicle with air suspension, you must turn off the suspension switch. find the reset switch auto parts store will tell you where its at

I had a 1999 Expedition. When you put the 4x4 on it raises the vehicle an extra inch or 2.
It may help to turn the 4x4 off. Two wheel drive is a lower air setting.
Your original pump may still be good. The problem can be in the front. The compressor is not filling the front suspension to the cutoff point. It will lift the rear after the front end is at running height. Since you lost the front end first, the remaining air bled out there and then the rear came down. Once you shut down the car the suspension was done, all the air was let go.
There are adjustment solenoids on the body but you need someone qualified to repair this.

rear suspension ride leveler mslfunction. Check Air compressor function.

hummer h2 hydraulic keeps running depleting battery.

the compressor is mounted down low in the left front behind the left headlight. it is powered by a large fuse in the under hood right side fuse panel. if the fuse is good.flip off the trunk suspension switch,jack up the rear wheels and check to see if the sencing arm that attaches to the rear end has fallen off it's pivot. if it is unattached it will drop both rear air bag pressures quickly.
good luck chris