MBP will not Boot after logic board out

I reversed over my bag about 5 years ago and compacted my MBP.. The insurance got me a new unibody i7 but a few years later I saw my old model as a water damaged unit on eBay. I knew the one I ran over still worked fine albeit with a bent chassis and shattered LCD so I bought it as a donor machine. Thing is after swapping out the logic board and pretty much everything else it worked fine.. I just had forgotten to reattach the tiny lead from the heatsink so the fans were on full all the time. So I pulled the logic board again to get at the trapped lead, cleaned off and re-applied thermal paste again etc. This time however it won't boot. CD drive whirs, fans startup, keyboard illumination comes on, I hear the chime and the HDD spins (but can't hear the read head move) and nothing else.. blank screen. I've tried an external monitor but again, nothing. I guess somehow I fried something on the logic board the second time it came out.. I've checked and rechecked all cables but i'm now at a bit of a loss...