If it's been sitting for awhile either out or in, the moving parts need lubricating. Especially the ball style conection on the push arm from the motor. You likely have to get on your back and reach up from under the step to access it. WD40 works good as a starter. When the step is free and can be moved with one person pulling on the step and the other opening and closing the door. When is is operating correctly wipe the extra wd40 off and spray with a silicone lubricant ( dries and lubs the joints and doesn't attract dirt sand etc..)
NB: The other problem could be you have the motor running or the key in aux position. There is an over-ride system that will automaticly close the step as a safety feature so you don't drive away with the step out. Check this as well when the step is working again to ensure the switches are ok.
Hope this helps a bit anyway.
B.