iPhone 6s Plus black and unresponsive even after display replacement.

My phone when black while I was reading. Siri worked for a minute until I tried a hard reset. Now I cannot get it to turn on at all. It doesn't vibrate or ring or make any type of sound when I charge it. I tried holding both buttons down many times for as long as five minutes. I went to the Genius Bar and they replaced my display, but it didn't work and they said I had to replace the phone. They also said that it is turning on and connecting to the iOS. There is no water damage, but it did take a bit of a hard shock about a week before it died. I really can't replace a phone right now. Any ideas?