Hard Reset NOKIA Lumia 520

First method (Windows Phone 8):

First of all, power off the phone by using the Power button.In the next step press and hold the Power key for a short moment.Release the Power button as soon as phone vibrates and start holding Volume Down key.When the exclamtion mark appears on the screen release held key.Afterwards use the following sequence of buttons: Volume Up - > Volume Down - >Power - > Volume Down.Congratulations! The hard rest has just been performed.

Second method:

1. Start by turning on the phone using the Power button.
2. Then go to the Main Menu, find and select Windows Phone Settings.
3. In the next step tap  About.
4. Afterwards tap Reset your phone to confirm.
5. Success! The device is ready to use.

Third method (Windows Phone 7):

1. Power off your phone.
   
2. Then press and hold Volume Down + Camera key + Power button.
   
3. If you feel that phone vibrates, then release only Power button.
4. When your phone reboots, then release the reset of holded keys.
5. Done!