I bought a lumia 520 last year and it failed within 14 months. Windows OS is unstable and fails like it used to be on windows desktops. This is a common problem in lumia phones.
When I took the phone to Nokia care center, I was given an estimate of Rs.3500. As the problem is due windows and it may fail again, I decided not to repair as there is no point in paying Rs.3500 every time windows fails.
I escalated the product issue to Nokia/Microsoft customer support center and the head of Nokia care but it is of no use. Avoid this phone unless you want to throw it away after one year