Microsoft has released a possible fix for low-end Lumia smartphones that have been bricked after installing an early Windows 10 preview build.

It seems a shortage of memory in the Lumia 520 and Lumia 525 is behind the bricking of some of the units, after owners installed a Windows 10 Technical preview build, named 10051, released through Microsoft's tester Fast lane.
​Microsoft issues fix for Lumia phones bricked by Windows 10 Fast lane | ZDNet