Did you update Nokia Lumia 520 or Lumia 525 (including the 526 variant) with the preview Windows 10 build? Then you should hold off on rolling back to Windows Phone 8.1, multiple users report that they bricked their devices doing so with the Windows Phone Recover Tool.
Microsoft has paused the availability of the update while the issue is being investigated. You can join the discussion, the Microsoft team is working with users trying to resolve the issue.
Some users are offering solutions, though it would be best to wait for the official team to resolve the issue. Also note that some users are having similar issues with the Lumia 920.