Could be the ribbon cable, could be the video board, could be the DMD. First connect J201, disconnect DMD power connector and see if it boots. DMD uses 12V and 5V, just like CPU board. If no, reconnect DMD power, disconnect J201 and test the video voltages.
https://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Testing_DMD_Controller_Power