iPhone 6+ won't turn on

I have an iPhone 6 plus that was working fine. I gave it to my cousin after I replace the screen and days later the new screen stopped responding. The only way it would respond to touch is if she would press on the screen very hard. Her friend ended up pressing on the screen very hard to the point it cracked. Then phone would turn on again after that but the touch was unresponsive. Later on, she slammed it on her bed and it turned off completely. It hasn't turned on since.

iTunes does not recognize it at all. I've replace the battery, the charging port and the screen and still no response. The motherboard however does get warm when plugged into the computer. What can be done? Any suggestions? Please and thank you!