Get access to the disk laser. There are plenty of online tutorials on how to do it. Once you have opened the console and found the laser, clean it off with isopropyl alcohol and a cotton swab. Then, before you close your console, put a game on the tray and turn on the 360 and if that does not work you will need to buy a new laser off of Amazon or Ebay or whatever and replace the laser.
Keep the system open when you test a game so if it doesn't work you don't have to open it back up again.