Reseat the RAM.
If both the power LED and HDD activity LED are constantly lit, then your RAM is not seated properly.
Well, that's what I've found from numerous experiences (4 or so when RAM was not seated properly, and countless where RAM was not working because it was overclocked, so wouldn't run and needed a CMOS reset).
Since you say you removed your RAM, I would say you didn't put it back in properly.