Maybe you did in fact burn (a)? I don't know -- and it sounds like maybe you don't know what's on that DVD, either. That's a tough one to sort out.
Where does "Disk Utility" come into the equation? Do you, perhaps, have the disk image of a DVD mounted on your Desktop? If so, DVD Player won't know the difference between that and a real DVD that's inserted into the DVD drive -- and it will auto-play it as if it were a DVD disk inserted into the optical drive. If this is so, simply eject the disk image, and you should be able to play a DVD from the optical drive.