Recently Microsoft made available the Windows 10 Technical Preview for Lumia 520/521 and 525. These are the weakest phones among the Windows Phone 8 gang with just half a gig of RAM. The update process went well for the early adopters, though issues arose when trying to roll back to Windows Phone 8.1.
Apparently lots of users, who didn't want to stick with the Windows 10 Preview got their Lumia 52x bricked after trying to revert back to Windows Phone 8.1. Some devices got stuck on a red Nokia logo, while others just went dead with nothing on the screen.
Microsoft has a dedicated Windows Phone Recovery Tool, which usually saves bricked smartphones. Unfortunately it doesn't help with the bricked Lumia 5 series. Microsoft now has a new version of this tool, which should help recover the bricked Lumia 520/521/525.
According to Microsoft the major issue was with the data writing size and speed. Microsoft basically lowered the image files and the writing speed. The company doesn't promise this updated toll will fix all bricked Lumia 52x, but it should work in most cases for those with a red Nokia logo and with the black screen.
Microsoft is still investigating the problem and is working to bring back to live all bricked Lumia smartphones. Meanwhile the Windows 10 Preview update is on hold until all issues are resolved.
You can download the new Windows Phone Recovery Tool from this link. Its version should be 1.2.4.