Library not loaded. Image not found.

Yesterday I resumed work on an app and found that I couldn't run the
app in the iPhone Simulator. As I recall, all I had done was gone to
bed and returned to my work.

Every time I tried to build and debug, I would get "Library not
loaded ... Image not found" errors, and the compiler would exit with

In my case, the CoreGraphics library couldn't be found.  I removed and re-added it and then Xcode complained about UIKit.

I found quite a few posts saying this issue might resolve itself, helped by cleaning, removing the build directory, closing Xcode, etc.

I couldn't get anything to work.

I had to revert to the previous copy of the xcodeproj project directory.  Once I copied it over the existing (corrupt?) one, it worked again.