I've loaded a project in Xcode 4 after not looking at it for a while. Xcode shows its progress bar and indicates in text that the project is loading, and it appears to finish, but then there are no files shown in the hierarchy or flat views. There is nothing showing in any view (except for one breakpoint in the breakpoints view).
Any ideas? Xcode gives me zero indication that anything is wrong, it shows me nothing.
I just had the same issue. After a few hours scratching my head and trying all kind of things, I finally found a fix: remove the whole package 'project.xcworkspace' inside the XXXXX.xcodeproj package.
More specifically, when you find the 'XXXXX.xcodeproj' file, right-click it, and select 'Show Package Contents'. This will get you inside the package where you will find the 'package.xcworkspace' file. It is best to do this while the Xcode project is closed.
Maybe that will work for you or for others with the same issue in the future...