Why isnt my iPhone turning on?

Remove the new screen.

Disconnect the battery. DO NOT remove it.

Press and hold power button for ~15 seconds.

Re-connect the battery.

Re-install the new screen, make sure it is properly seated. DO NOT yet re-install the small shield covering the screen connectors.

Try to restart your phone, and if needed charge it for 20 minutes first to make sure the battery is holding a charge.

If you still get nothing, repeat the above BUT install the old original screen back, in case you still have it and it is still working. If this time the phone restarts, you were trying to install a defective screen, get a new one.

If still nothing happens, let us know to help you figure out the next possible step.

Notes:

A/ As an additional measure, before re-installing the screen the first time, you can clean the screen connectors on the board using a cotton swab or a very soft brush dipped in 90%+ isopropyl alcohol.

B/ Use this guide, step 11, to place black the screws of the shield exactly where they should be. Do not mix them up or your risk permanently damaging your iPhone.