Is this slide electric driven or hydraulic driven?
If hydraulic, go outside, under the slide and take a look at the hydraulic ram cylinder that comes out of the frame and look at the end of it where it attaches to the square tube. Is the bracket there broken? Look for a square bar that goes from the drive end back to the other end of the slide where it spins a gear to push that rack out. Anything look funky there?
If it's an electric slide, is it a Norco-BAL cable type (visible cables outside on the sides of the slide) or is it a Schwintek gear type?
If Schwintek, then you have a motor behind the slide fascia, one on each side. It's possible that one motor is bad, disconnected (electrically) or disengaged (mechanically). There's a good manual on this on the forum under Tools - Sliderooms.
If Norco-Bal, it could be the cables on one side came off a pulley of some such. This mechanism is usually under the front end of the bed. Here again, some good manuals are online for this.
Give us more info to help you with ideas.