I always have this problem with my XBOX 360 halo 3. Here is a sounds-like-abusive-suggestion but it works to those who bought it from other people.
1) Open the tray and push it closed with your finger.
2) As soon as it says, “Reading”, Beat above the tray as hard as you can.
3) If it didn't work the first time, press the red button underneath the tray.
4) Repeat the steps above.
5) If that didn't do the trick, try cleaning the disc, buying a new one, or cleaning the laser with a soft, damp cloth with rubbing alcohol.