Have you replaced anything on the xbox, or done any other work to it? If you replaced the original DVD drive with another one that is not "married" to the motherboard, it will not play game discs. The motherboard and DVD drive share a key, it is different for every system. If the DVD drive was replaced, here's a write-up that gives you some options on how to put the original key on your new DVD drive so that you can play games on it again.
http://hardandsoftgaming.wordpress.com/2011/03/12/replacing-a-faulty-xbox-360-dvd-drive-firmware-flash-or-circuit-board-transplant/