On boot no chime and no screen, but HD churns

Hi,

My Macbook early 2006 has died. It shut down a couple of times while working intensively. Soon realised it was overheating because of stuck fan. So I opened it up following ifixit guides and cleaned fan. To make sure I turned it on and off a couple of times before reassembling top case. Everything worked great.

Proceeded to reassembly and then nothing. It just doesn't turn on. When pressing button the screen flashes for a second and goes black, then disk churns as usual but no chime, no bootup. Caps lock unresponsive but front led is on (as if in sleep?)

Tried PR reset, SMC reset, target disk mode... nothing. Neither does booting from a CD (though drive spins as usual) work; now boot CD is stuck in too. Unable to run hardware test.

Took it to genius bar, they hooked it up to external display and no video coming out. They couldn't determine what was wrong after repeating my steps (pr reset, etc) and some others, like trying to netboot it. Nothing. They suggested sending it for flat fee repair.

So now I want to try and repair it, but I don't know where to begin? Main board? Memory? Did I fry it during re-asembly? Where do I start? I've searched everywhere for answers, but nothing so far. MacBook only boots fully occasionally, otherwise black screen was close, but was unresolved. Any hints of things to try or tests I can do to figure out which component fails would be appreciated.