As those consoles get older, the laser loses power and will start so say "unrecognized disk" or simply "open tray" when you insert a game. It will require for you to open it up. Here's the video I used:

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Also, I've read that you can hit the top off the console repeatedly right after inserting the disk and that sometimes works as a temporary fix. This, of course, risks breaking something else and making it worse.