Step 1 Reset your phone in the Recovery Mode
When your device is in the Recovery Mode, the iTunes logo and USB cable appear on the screen.
Install iTunes to your computer and then connect your phone to the PC via USB cable. After your iPhone is detected by iTunes, click the "Restore" button to put your iPhone into factory settings.
Step 2 Restore your iDevice
When you finish the files syncing with iTunes, you will see the interface below. Select "Restore iPhone" to finish the original settings and then tap on the "Restore" icon to enter the next stage.
Step 3 Retrieve data from iCloud backup
After resetting your device, you are asked to do some basic settings to the phone when turning it on. Then, you will see the following screen. Tap on the "Restore from iCloud Backup" and then sign in your iCloud account to restore data to your device.
Follow the steps above, your phone data will be back to your iOS device. But I have to remind you that this approach is based on one premise: all you iPhone information has been backed up to iCloud. Therefore, it is necessary for you to do some backup constantly.