nbox is blank in front but can hear hard drive.

I was showing someone how to flash the nbox to have multiboot. after doing this and trying to show them the option to get to bu4. the box came back dead. the box has a a19 and a20. if i change the jumper to a19. it will come up uboot 1.5.1.xxxxx and not change screen but when I have the jumper on the a20 its appear blank. any suggestions what i can do. i now have the box opened and the mother board out to check the cable.