Nokia Lumia 520 hard reset:
if you use windows 8
* Turn off your phone. Press and hold the Volume down and connect the charger
* you will see exclamation mark (!)
* Press below keys exact order
1. vol up
2. Vol down
3. Power
4. Vol down
phone will be reset will all content except SD card
If you use windows 7
* Turn off your phone
* Press and hold volume down and camera key
* press and hold the power key
* you will feel an vibrate so you can release only power key and keep pressing vol+ cam key until phone reboots
phone will be reset will all content except SD card