The same issue happened to me and I found that by re-seating the interneal connectors for the DVD and the HD solved this issue! I spent several hours reading up about this issue and it was never mentioned anywhere. Based on what I had read I thought had to be my logic board but it WASN'T!
I read in several places that if the following conditions occur then it 'had' to be the logic board
(1) No startup sound
(2) Solid power light (not flashing)
(3) PRAM and SMC/PMU reset don't work
(4) RAM is compatable and correctly seated
If you can't get your system to work with the above you should check any internal connections to see if they are causing your issue, it was for me.