I'm not an expert, but I think is because of the RAM .. if there is no RAM, there is no CPU, and if is no CPU there is nothing .. no response. When I first upgraded my RAM, router didn't start .. all LEDs went on. Then I reheated all connections and it worked.
Your problem might be a cold solder joint too .. recheck connections (even though it worked first time you tried .. a little shake and a bad joint won't work). Though, I'm not sure what does it mean the fact that your power LED doesn't turn on. Maybe you got in a nasty coincidence and your AC-DC adapter went faulty .. also check the 5V power input port under load.
edit:) .. I just noticed that the thread was started almost 3 weeks ago. Hope is not to late ...
