Depending on the manufacture of the slide, I know of two manufactures and each has a different procedure for overriding the motor.
Schwintek has two motors and they must be disengaged from the slide. Once disengaged you physically push the slide either in or out.
Happyjac there is usually an override switch to disengage the power to the slide motor and then you have a manual crank that is used to move the slide.
I'm sure other will chime in with recommendations but in the meantime you may want too google it once you know which slide manufacture you have.