This indicates that your CPU board has no activity. At this point, I would remove all ribbon cables from the CPU board and power up and see if the normal activity light flashes. If so then reinstall one ribbon cable at a time repeating the test. Note that one cable goes to multiple boards so you will need to isolate those as well. Please report back the findings.

If the CPU does not get activity even with no ribbon cables, you could verify voltage at the input to the board for 5v and 12v. If that's ok then you may need to repair the CPU board or just replace it.

Markmon's offered you a good start.
As he suggested, test the 5VDC power on the power/driver board. It should be above about 4.93VDC or so.

Next, in addition to removing the ribbon cables, remove the 4 lower switch matrix connectors. All that should be left is the input power connector. Power up. Has the LED activity changed?

Any alkaline damage on the board?

With power off, press firmly on the ASIC (square chip in the board center). Reboot. Any change?

Report back soldier.
--
Chris Hibler - CARGPB #31
http://www.Team-EM.com
http://webpages.charter.net/chibler/Pinball.htm
http://www.PinWiki.com - The new place for pinball repair info

I know this is terribly old, but I found it when I had a stuck on LED201 on JY tonight. Anyway, for anyone in the future, just FYI, mine ended up being almost undetectable corrosion damage in the socket of G11. Pulled socket, neutralized board, cleaned, buzzed out continuity points, soldered in new machine pin sockets and dropped in 6809, good to go!