Hi guys I am new here so hello to everyone lol

I have a Dual 1.42 ghz power pc g4 with a superdrive. Just recently I have been unable to write anything on cd at all. I tried backing up all my documents email etc on cd and it writes the cd then finishs... nothing happens I eject the disc and put it back in and its says invalid disc eject it or ignore it, it doesn't seem to work with normal backup or itunes either I still have the same problem. Has anyone faced this problem before or have any idea about what to do about it? Any help would be really appreciated.