If the display stopped showing video all together after you started reseating cables, please make sure
(1) The Whitestar display board (large one on the rear of the display) is receiving 5V on the two-pin power connector
(2) The two ribbon cables plugged into the Whitestar display board are seated correctly with the red stripes closest to the corners of the board.
(3) The ribbon cable plugged into the ColorDMD controller board has the red stripe closest to the buttons
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EDIT: Here is a correction to a some things I posted above for SST (was thinking about DE factory boards and SEGA is different). Below is schematic for the original power supply board. 12V unregulated comes out on J16-3, fed by the 12V bridge in yellow. 5V comes out on several pins fed from the 5V regulator in red. It is driven by a different and dedicated bridge (in blue). So for the factory board, 12V should not impact 5V at all.
Unfortunately I can't track down a schematic of the Rottendog SDB004 to compare. However here's a discussion of the known problem that exists for the Rottendog DE power board. https://pinside.com/pinball/forum/topic/rottendog-data-east-power-supply-board-design-issue
Unlike the DE board above , the 12V regulator is not on this board, it's on the audio board. So the Pinsound and ColorDMD are both directly pulling current from the same 12V bridge and the output is unregulated. Large current surges could result in voltage fluctuations at the output of the bridge.
It looks like the 5V regulator on the RD SDB004 might have a jumper option as to which bridge is driving it. You should check to see if this is jumpered to connect to the 12V bridge. If so, voltage fluctuations on 12V could be causing 5V to swing and could result in odd logic behavior or data communication problems between the boards.