What is the best way to clean/repair a PS2 disc?

If this is indeed even my problem. I have had a PS2 for a while but rarely used it. My son has finally gotten to the age where he is starting to use it, and he got several games for Xmas. There is one that he has been playing a lot lately, and he brought it to a friend’s house today. When he brought it home, it didn’t look like it was any more scratched or dirty than it had been, maybe some more finger smudges, but I wiped the surface clean w/ a soft dry cloth first. Anyway, there was a loading screen that came on and it just kept sitting there, and finally we got a disc load error, and it said checking disc, but went right back to that loading screen and kept trying to load but went nowhere. I don’t know if it really is the disc, if it’s scratched, or just has some dirt I can’t see or if it’s the laser on my PS2 or what. What do you hardcore PS2 veterans do when a disc stops working?