I'm assuming you have a Norco Electric Slide. Their manuals can be downloaded at:
http://www.norcoind.com/bal/downloads/manuals/index.htm
It seems that cables can stretch, chains can become loose, & a world of other problems. Visit the above site for their excellent
? Accu-Slide Designer's Guide
? Accu-Slide Installation Manual
? Accu-Slide Service Manual
Looking at the secret GS schematic that documents the NORCO slide, I see that there seems to be a single, double pole, double throw switch that controls the motor (and the direction of rotation). The operator on the switch is likely (as stated by earljan) pushed by some arm associated with the slide mechanism. I can't tell any more from the drawing. Open your slide will someone observes from underneath. Let us know if you find the switch. The problem is possibly an adjustment-thing.