Xcode 4 shows no files after loading project

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.