If you've tried all the prior fixes, and nothing has succeeded, you probably have a hardware problem. Such symptoms may include a persistent black screen or an iPhone that refuses to appear in iTunes. Here's how to proceed:
If your iPhone displays the message "Repair Needed. iPhone cannot make or receive calls," it requires servicing.
If the iPhone screen displays a yellow triangle with an exclamation mark, it also likely needs to be repaired.
Otherwise, confirm that your problem is specific to your iPhone. For example, if the symptom is that the iPhone will not charge from a particular charger, make sure the charger itself is not the problem (try a different charger if possible) and that it is connected to an active ...