Eject the DVD and see if it is the same one that was there when you took the car in.

If it is, then pull the fuse for the radio under the hood and under the dash on the driver's side. Reinstall the fuses after a few minutes. That will force a re-load of the sofware and if it is the version that has the override in it, it should work again.

If they replaced your DVD without you knowing, then I would go back and demand that they give it back to you.