Usually cleaning the laser works.
I cleaned my first PS2's laser which I saved for some reason...about 4yrs+. By this time I was on my 3rd one...I cleaned it and all was well again.
But if that doesn't work then the laser is bad or rail slide assy is out of alignment. If its an older PS2 like u mentioned, u can also turn the white gear (about size of a quarter if its there) a couple notches one way or the other. Just make sure u mark it before moving it. Gear is at back of CD unit under the CD player cover plate. Adjusting this gear adjusts the laser angle. Sometimes a click or to will get em to read. But some of the other PS2 discs (silver backed) may not work.