I'm going to venture a guess you have an in-floor, through frame slide out (see link):
First thing is to pull the slide control switch and ensure there is power to it, and that the switch is good. Usually four wires, two have 12VDC (+ and -), the other two go to the drive motor. The switch will reverse polarity to drive slide in/out.
Then check drive mechanism. There's one electric motor that drives a linear actuator to push slide in and out. The slide has two (toothed) rails sitting on spur gears which are connected with a torque tube so they drive together. OK, so after that mouthful, you can open the RV belly and apply (or check for) 12VDC power directly to the motor to see if it drives. There is a shaft on the backside of the motor, protruding through the frame (opposite side of the RV from the slide). You should have a hand crank to manually move the slide (by manually turning the motor shaft with the hand crank). Could be bad motor.
Let me know if this helps, or you have different style slide.