The most common reasons an iPhone 4 won’t turn on is it either needs to be rebooted because it has crashed, or the battery needs to be charged. Finding out if your iPhone 4 won’t turn on because of either of these issues is the best place to start to see if you can get it to turn back on. The first step is to charge your iPhone 4 for at least an hour or more. It’s best at this point to plug it into a wall outlet if possible. Also, before you start charging, check your cable and input to make sure it is free from lint or other debris that might not allow the charge to come through.
After you have charged your iPhone 4 for at least an hour, or at least four hours if the iPhone was completely dead, unplug it and hold down the Power and Home button at the same time until you see the Apple logo appear. This will force your iPhone to reboot and this should correct the issue if your iPhone 4 stopped turning on because it crashed. After it reboots and has been charged, it should operate normally.
If your iPhone 4 still won’t turn on even though you have charged it and rebooted it, then there could be some other issues. On rare occurrences a battery will fail to charge anymore, in which case you would need to replace the battery. Another issue is if the USB charger is no longer working. If you have another cable and can charge the iPhone a different way, you can try that to see if it will charge.
If your iPhone 4 was dropped, has water damage or some other accident happened, it could be that the iPhone 4 is broken. Also, sometimes it’s just a bad iPhone and if that is the case Apple is pretty good about switching the iPhone out without charging you for it. You can contact the Genius Bar if you have tried everything and your iPhone 4 hasn’t turned on. If it is under warranty, then you can probably expect that the issue will be fixed and if not then the Genius Bar might at least help you find out what you might be able to do to fix it yourself.
Source OXS Daily