I think I recognize this wellknown, longstanding bug:
- https://web.archive.org/web/20100617142305/http://connect.microsoft.com/VisualStudio/feedback/details/105511/find-in-files-says-no-files-were-found-to-look-in-find-was-stopped
The workaround mentioned there worked for me when I had this problem
I found a tip to try pressing Ctrl + ScrollLock to fix it, and it worked.
I think most people report success with the (Ctrl-)Break key, but some say (Ctrl-)ScrollLock worked instead