'Fraid it'll have to go back in the shop again!
Many times you have to select DVD as the source, but also have to select where you want that signal sent --- The front TV. That's the way the B.O.M.B. (Box Of Many Buttons) usually works. Check what type of output cable is connected to the DVD. If it's HDMI, your TV should have an input port for that. If it's coax cable, it might be going through the B.O.M.B.
Does the TV work from antenna or cable connections? You could try that. Also, if all else fails, use a separate cable and hook the TV directly to the DVD player until you get it figured out.