iPhone screen refuses to work even after screen replacement and restore

I have had my iphone 6 since february, no problems. Recently, I did drop it and break the screen(it was fine though, I could still see everything fine). My mom replaced the screen a week ago and everything worked perfectly fine, until about 3 days later.

I went to plug in my phone a few nights ago and before I could even lock it, the screen went black (ps - the phone wasn't dead, it had 40%). My initial response was to hard reset it. Nothing. I could still hear the phone vibrate. So, I tried factory resetting it. I tried restoring it with iTunes about a dozen times. Still nothing. However, I know the phone reset itself, as the phone had voice control on and was reading the screen ("Hello, press home button to begin"). It was also saying "Screen dim."

So, my next step was to have my mom look at the phone, as she is a technician. She couldn't find any problems with loose cables or anything like that. So we figured it must've been an issue with the LCD we ordered, and we tried a different company.

She installed the new screen today and still nothing. Whenever she would reset it, the Apple symbol appeared for a very brief time, but it was very dim and flickered.

We have tried just about everything. Disconnecting the battery, cleaning the interior, reconnecting cables, everything. Nothing even looks like it was damaged from me dropping it or anything like that.

Please help, I don't know what I should do. Are we dealing with a software problem or hardware? Or is the phone just a brick now?