iPhone will not turn on

Are you sure the phone is "dead/not turning on?" May be the phone is turning ON but the screen isn't.

Charge the iPhone for 20 minutes using a known good cable and charge, to make sure the battery is holding a charge, then connect it to a computer. Is it recognized by iTunes?

- If not, you phone could be "dead" but not lost. Don't panic just yet. Let us know and we'll figure out what's next.

- If yes, iTunes sees the device, then here are some of the most common issues:

A/ Phone needs power cycling and/or rebooting:

A-1/ Reboot: press and hold home button + power button for ~12 seconds. Try to restart. Does it turn on?

A-2/ Power cycle your phone: lift the screen, disconnect the battery connector (DON'T try to remove the battery), press and hold power button for ~15 seconds. Reconnect and test. If screen starts, reset the time and date then reboot again.

B/ Connectors need cleaning and/or reseating:

Disconnect the screen from the phone, clean the connectors on the board (and the screen cables, but gently not to damage them or splash the screen) using a cotton swab dipped in 90%+ isopropyl alcohol, dry, reseat the screen cables gently but making sure they are firmly connected. Restart and test.

C/ Screen assembly is dead: because it came defective or you pulled on the screen cables while installing assembly. Try the old screen back, or a new known good screen, and see if it solves the issue.

D/ Pry damage: if you reach this point without having solved the issue, let us know, so I can point you to the location on the board that you need to examine for potential pry damage.

Good luck, and let us know what you find out.