iPhone 4s won't turn on after screen replacement

I cracked my screen pretty badly a few months ago and finally got around to fixing it. I borrowed some iFixit tools from my friend, and used the iFixit screen replacement guide to take off my screen. I removed the back plate, the batteries, the speakers, the logic board, and everything else in order to get to the screws to take off my screen. My cracked LCD fell off in pieces, and I used tweezers to pick out the small black shards that were still stuck onto the adhesive of my phone.

After carelessly putting in my new, $60 screen, I realized (after turning my phone on) that the ribbon cable that picked up the touch of my finger had ripped, rendering it useless. Other than that, everything else worked fine, so I figured I just needed a new LCD.

After waiting another week for this new LCD, I very carefully reinstalled the new LCD in the same way I had before. However, after putting everything back together, the iPhone would not turn on. I initially thought that the LCD was faulty, but after trying to hold the home and the sleep button to turn it on, there was no vibration, and I could tell it wasn't on by calling it.

I then tested if my battery had somehow broken in the repair, but I plugged my own battery into another iPhone 4s, and that iPhone worked fine.

Please, if any of you have any insight or idea on what the problem is and/or how I could fix it, I would be extremely grateful. Thanks.