IPhone batteries are designed with longevity in mind, and it should retain up to 80 percent of its original capacity after 400 full charges, according to Apple. If your iPhone no longer holds a charge for the same length of time that it once did, you may need to invest in a replacement, but, if your battery simply refuses to charge, your problem is more likely related to your firmware or to an external factor. You can pinpoint the exact problem by doing a bit of troubleshooting.
Leave the iPhone plugged in for several minutes. If the iPhone appears to not charge, at first, the battery may need time to restore before presenting any screen display. If the battery is completely dead, even a connected iPhone may remain unresponsive for a short time.
Reset your iPhone. An inability to charge may indicate a frozen or malfunctioning operating system, which you can sometimes remedy with a hard reset. Hold down the two main buttons on your iPhone ("Home" and "Sleep/Wake") until the Apple icon appears.