If your PlayStation 2 Slim isn't reading discs, there are a few things you can try to fix the issue. First, check that the disc is clean and free of any scratches. If it's dirty or damaged, cleaning it or replacing it may solve the problem. If the disc is in good condition, try resetting the PS2 by unplugging it from the power source for 30 seconds. Once it's been reset, try inserting the disc again.
If your PS2 still isn't reading discs after trying these troubleshooting tips, there may be an issue with the hardware. The optical lens inside the PS2 Slim's disc reader may be dirty or faulty. You can clean the lens yourself with a soft cloth, but be careful not to damage it. If cleaning doesn't work, you'll need to replace the entire disc reader assembly. Fortunately, this part is relatively easy to find and replace on your own.