I replaced the upper case and now it doesn't boot

Hi, I had a nonfunctional keyboard and power button on my late 2011 MacBook Pro unibody, so on the diagnosis at the Apple Store, I went ahead and replaced the upper case, thinking I could get it working again and sell it. I even had a buyer lined up! Unfortunately, somewhere along the way, something got screwed up, and now it doesn't work at all.

Any ideas on how to figure out what the problem is exactly and how to fix it?