how funny!!! well not really, but I had the same problem. But for me it is the keyboard ribbon. I've taken apart my computer many times and that gives me the most trouble every time. If it isn't plugged in, the power button won't do anything. so Try to reinsert the keyboard cable, hopefully that solves your issue, that did it for me.
and the thermal paste cooled my comp by like 10C while watching netflix!! amazing