If your iPhone seems to be playing up or you need to get something back that you accidentally deleted then you will need to reset and restore your device to an earlier state.
1. Check your backup
Before you start – make sure that you create a backup. The easiest way to do this is through iCloud. To check it’s enabled, open the Settings app, select iCloud, followed by Storage & Backup. Here you’ll want to make sure iCloud Backup is turned on, then take a look at when the last backup was at the bottom of the screen. If backup was not enabled previously then you won’t be able to retrieve any deleted items unless you’ve backed up to iTunes on a computer, as described below.
If you don’t use iCloud or have a device that’s not compatible with Apple’s storage solution then you’ll have to create a backup using iTunes on your computer. To do this, connect to your computer using the USB cable and then open iTunes. Click on your device name in the top right and check the Summary page. Here you will see the Backups section, which tells you when the last backup was created. If no backup is listed then click Back Up Now. When you reset, it will only restore items which are currently on the device – unfortunately you won’t be able to get back any recently deleted items. 
use iTunes on a computer to create a backup of your device.
Use iTunes on a computer to create a backup of your device. Once a backup is created using one of the above methods, you’ll want to reset your device. Open the Settings app, select General, then Reset. From here you want to hit Erase All Content and Settings. If you have set a passcode, you’ll need to enter this to continue. Once you have, the device will wipe everything – a progress bar will let you know how it’s going.
If you use a passcode then you’ll need to enter it before you can start resetting your device 
2. Setup process
Once this is complete, it will enter setup mode, just like a new device. During this process you’ll be asked if you want to restore the device. If you’re using iCloud, select the Restore from iCloud option followed by your choice of backup. If using iTunes, select Restore from iTunes and then choose a backup from your computer screen. This will then restore settings, apps, and deleted items on to your device. Note that this may take some time, so make sure you’ve a bit of free time before you begin the process of restoring your phone.
If you restore from iCloud then you’ll need to wait for the apps to download.
3. Recovery Mode
If you’ve forgotten your passcode then you’ll need to enter Recovery Mode. Before you start, make sure your computer is running the latest version of iTunes. To check, open iTunes and select the iTunes drop down menu, followed by Check for Updates on a Mac, or Help on a Windows machine, followed by Check for Updates. This tells you if you’re up to date and if not, how to download and update iTunes.
You can now enter Recovery Mode. Start by turning off your iPhone completely by pressing the Sleep/ Wake button until the ‘slide to power off’message appears. Confirm by sliding to power off. Once it has shut down, press and hold the Home button and connect the phone to your computer using your USB cable.
iTunes will then pop up with a message stating that the phone is in Recovery mode and you must use iTunes to restore. You can give your thumbs a rest and let go of the Home button now too. Now, click the Restore iPhone option and hit confirm. If you were not using the latest version of iOS, then be aware that it will also update to this.
You will then have to wait for the download process to finish and then the setup process will begin. During this you can decide to restore from a backup, or if you’re worried something was wrong with your previous setup, then choose to setup as a new device. Once it’s been set up, you can open the Settings app, select iCloud and sign in to your account. This will transfer contacts, notes, and other items from iCloud back to your device.
4. All re-set
Now your device has been reset you’ll be able to use it as normal again. This is a time-consuming task but it’s one that’s worth undertaking before undertaking a lengthy trip to an Apple Store, or throwing your phone at a wall in frustration, no matter how tempting that may be.