Troubleshooting dead iPhone after fall

Friend of mine dropped his Iphone 6s, landed hard and busted the screen, we couldn’t get it to turn on with the original so I installed an original 6s screen I use as a testing screen, didn’t work, phone won’t turn on, tried a tester battery also and no joy, any ideas? Suggestions on any further troubleshooting? Everything inside the case is sturdy and fixed in place, no bends nor dents, connectors for the screen look good and no bent pins.

EDIT: tried both regular reset and hard reset, none work.

Any ideas fellow fixers?

cheers