1. The most useful way is hardware repair, it can solve any problem, check here to get hardware to fix iPhone won't turn on.
But sometimes, your iPhone doesn't need to hardware repair, so you can read the next ways to slove it.
2. Charge Your Phone
It may sound obvious, but be sure your iPhone's battery is charged enough to run the phone. To test this, plug your iPhone into a wall charger or into your computer. Let it charge for 15-30 minutes. It may turn on automatically. You may also need to hold down the on/off button to turn it on.
3. Restart iPhone
If charging the battery didn't turn your iPhone on, the next thing you should try is to restart the phone. To do this, hold down the on/off button at the top right corner or the right edge of the phone for a few seconds. If the phone is off, it should turn on. If it's on, you may see the slider offering to turn it off.
4. Hard Reset the iPhone
Try a hard reset if the standard restart didn't do the trick. A hard reset is like a restart that clears more of the device's memory for a more comprehensive reset.
5. Restore iPhone to Factory Settings
Sometimes your best bet is restoring your iPhone to its factory settings. This erases all the data and settings on your phone and can solve a lot of problems.
6. Put iPhone Into DFU Mode
In some situations, your iPhone may not turn on because it won't boot up. This can happen after jailbreaking or when you try to install an iOS update without enough battery life.
7. Reset Proximity Sensor
Another rare situation that causes your iPhone not to turn on is a malfunction in the proximity sensor that dims iPhone's screen when you hold it up to your face.
