iPhone 6 went dead after screen was replaced.

Phone screen was replaced Friday. iPhone turned off Sunday afternoon and is now completely unresponsive. I've tried everything to get it back; hard reset, leaving it on the charger for hours, blowing out the charger port to make sure it was clean, and tried multiple chargers. Then i took the phone back to the store this morning and the guy claims that its not the screens fault that the phone went dead even though before the screen was just cracked and the phone worked perfectly fine. So i think the guy is trying to get out of fixing my phone, what can i do to get him to fix it? And I'm saying guy but its a PC repair store that is a certified Apple repair store.