I assume you're talking about the original Playstation, and not the slimmed down PSOne. Unfortunately, this is a common issue with the original Playstations, one day they just stop playing, but there are a few things you can try to extend the playing life.
Use this [guide|1595|guide] to open up to the optical drive. Be careful, its only held in with the ribbon cable. First I would try cleaning the lens. If that doesnt work, try opening up the module and using a penetrating lubricant(like liquid wrench), lubricate the inside of the spindle and the screw track and rod for the lens.