ANSWER:

I encountered the same problem today. Restarting XCode did not fix the problem for me. I was able to get things back to normal by using “Delete” option of “Derived Data” of the project that can be found under Organizer. Organizer says that, “Derived Data includes index, build output and logs”. I guess that either index or build output was causing this issue.

by Praveen Kumar from http://stackoverflow.com/questions/10268408