Same problem.
I had to change the magsafe board because it wasn’t charging. Everything seemed to go okay, but now the MB doesn’t boot. The charging light comes on when the magsafe charger is connected, so that problem might be solved.
I’ve tried the suggested tape method to slide the keyboard cable into the connector better, but still no go. So, I want to ask just HOW FAR the cable has to be inserted? I can just see a thin line of metal contacts on the cable when it is inserted. Should it go in far enough so that you DON’T see the contacts?
On the iFixit page about changing the top case, the foto in the take apart shows the thin line of contacts just before removing the cable. See STEP 14 here: [guide|2185]
I’ve fixed a fair number of MacBooks, but if the problem here is not the cable then I guess I fried the motherboard.