When the jack is fully extended or lowered you will get a clicking sound. That is the built in safety that releases the motor before it comes to the limit of the jack.
Assuming this is the condition you should be able to move the jack in the other direction by just choosing the other direction on the switch.
If the jack clicks in both direction from its current height it may be jammed against the stop inside the jack tube. If this is the case remove the top cover and use the manual handle to release the jam. Once released you should be able to test it by using the motor while the cover is off.
If the jack was jammed against the stop in the tube you will need to reset the limit switch in the motor to prevent this from happening again.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles