After a screen replacement, why am I only getting picture sometimes?

I recently received an iPhone 6s screen replacement kit as a gift. I followed a YouTube tutorial. Some screws were lost along the way (a few from the motherboard plate, the two screws on either side of the charger port, and a few that hold the metal plate) but I've ordered more to replace them. I also ended up scratching the metal plate with a razor trying to remove some tough screws. When connecting the new screen to my phone, I mistakenly tried attaching the wrong ribbon cable for quite a few minutes. After realizing my mistake I looked closely at each prong but nothing was bent out of place. I got everything back into place and turned the phone on. It worked, but only for a little. I pressed the lock button so that I could tell the time, but my screen stayed black. However, my phone was still on because I could hear myself getting a phone call. Minutes later, it worked again. And then it didn't, again. I disconnected my battery for about 45 seconds and turned my phone on again. It was on, but black. Moments later my screen worked again and now I'm typing this on the phone. I'm sure I did a lot wrong but my local prices to replace a screen are about as much as replacing a whole iPhone 6s. Help!!!