Back ups stopped working.

I am currently using a brand new 24 inch imac with its built in dvd burner to burn 360 games. My friend helped me mod my xbox and helped me back up my gears of war 2 and burned a copy of it on his pc. After i came home i backed up a copy of one of my games using iburn360 and a verbatim dvd r dl. At first the game worked and i played it for awhile but today when i tried to play it, the xbox would just say open tray. The copy of gears of war 2 that my friend burned for me still works, but neither of the games i have burned work.