Put the disc in, making sure to note how the disc is oriented in the tray. Close the tray, and listen to the drive. When it's done, open the tray, and see if the disc has spun. If not, and you didn't hear the normal sounds, the disc drive is broken. You will have to send it to Microsoft to be fixed, which sucks since those places specialize in RROD. Once it gets to the facility, RROD Xboxs are almost instantly sent back repaired or replaced, whereas when I sent in my disc drive broken Xbox it took a week just sitting there (not including the week it took there and the other one back).