There is a big history of this with the PS2, basically the laser head has gone tits up, it usually starts off sporadically not reading then just gets worst until it won't read anything. There were some court case's against Sony and they agreed to take the units back and repair them free of charge but there were still machines that screwed up a second time, this is going back a few years (like about a year after they first came out). Mine started to do this a couple of years back then seemed to be ok, it hasn't messed me around for about 10 months now. (Now I've said that it will probably pack up tomorrow )
I'll see if I can find some more info on this, this is only from what I can remember from back then but it was a big story in the news.