if you don't want to mess with phone ( as it voids warranty ) try this: If you have a nokia lumia 520 and cant use lock and unlock ( also power off/on button), plug into charger, the phone will come on auto, go to settings>display and touch>touch>click option to double tap to unlock screen. You can now access phone when it goes on lock screen by double tapping any were on screen. This is a common fault with them. However you cant turn fone off doing this, but at least you can use phone. my button suddenly went on mine today ( common fault due to lead based solder apparently) and did research and didn't help but tried different settings and found this out, hope this helps you if you have same issue, ( although I did do the bit of plastic method and it does work)