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.
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.