when you open the "back box door" to expose the circuit boards in the back box, just look at the back of the DMD assembly. There will be 3 cables/wires of various style and width. If you carefully disconnect and reconnect each one, doing this one cable at a time AND reconnecting them in the same direction that they came off, you should be OK, that is what Chad is most likely referring to. Be sure to do this with the game powered off and unplugged from the wall just to be safe. Plug the game back in and turn it back on to check if the display comes back on.
If not, turn off the game and unplug from the wall to be safe. Follow the path of each of these DMD cables/wires to find where they connect to on the main circuit boards. Repeat the same procedure to disconnect and reconnect these cable/wires at the main board side but use plenty of care as you may end up loosening other cables and wires accidentally.
While you are at it, check the battery holder to be sure that they new batteries have been installed in the correct position. Also look for battery corrosion or broken battery terminal clips. Checking the battery holder for corrosion is a good idea and cleaning it up before it causes damage is a must. Many people install remote battery holders to relocate the batteries away from the circuit boards.
Chad is correct about the game probably wanting coins or credits to start. You'll have to go into game adjustment mode to return the game back to free play once you get the display dmd working again.
You could trigger the coin switch inside the coin door to simulate adding coins or just even drop a couple of coins through the slot to be sure that the game will start up.