There might be a few causes of the problem. We've decided not to cover them in this article, rather give you possible solutions right away.
To resolve the problem, you can take the steps described below. However, if the issue persists even after that, consult Apple customer service, or visit an Apple Store for in-person support.
Restart the iPhone. Now, that may seem too easy and kind of obvious, but restart can fix many things: poor cellular or Wi-Fi connectivity, app crashes, and other routine glitches. It is the simplest workaround which only takes a couple of seconds.
Use the Recovery Mode. Another simple procedure that can solve the problem in some cases. In the Recovery Mode, you can connect to iTunes and install the latest version of iOS or a backup of your data.
Use DFU mode. As you put your iPhone in the DFU (Device Firmware Update) Mode, the device stops halfway through the startup process. Here you'll have several options: connect to iTunes, restore the iPhone, load a backup, or start fresh. It might take some time to master using DFU Mode, but this method proves to be effective.
Overall, these advises should help you to get your iPhone back to normal.