To further explain on anijet's answer: the slides move by a hydraulic ram pushing on one end of the slide; each end has a gear pack (a toothed gear [pinion] that rides on the toothed track [rack] that you see under each slide and disappearing into the frame. The gear packs on each end are connected to each other by a long torque tube and the tube is held on by shear bolts. Apparently, on the end that does not want to move, the shear bolt is gone (sheared). You will need to cut into the belly cover underneath the trailer starting at the frame and moving inward. If you make the cut about 18" wide and leave the inside end attached, you will create a flap that will be easier to replace than if you cut the inside end also.
Then, having made the cut, you can see the gear pack and the torque tube. There will be a bolt hole on the end of the tube that does not have a bolt in it. When you make the cut, the old bolt will probably fall out. Replace the bolt after lining up the hole in the tube with the hole in the gear pack shaft. You will have to align the opposite ends to make sure the slide goes in/out both ends at same time
When you get the flap open, the operation will become somewhat apparent and better to understand than just my trying to explain it.
I did both ends of the door side slide on my '05--not terribly difficult, just tedious and working on your back trying to sit up into the frame are with your arms isn't easy.
See if this link helps you with some pictures:
https://www.lci1.com/through-frame-slideout
Joe