Error message when compiling in Xcode 4

As of today I can't compile my iOS Xcode project anymore. Immediately a message box pops up saying:

The run destination iPhone 4.3 Simulator is not valid for Running the scheme 'MyApp'. The scheme 'MyApp' contains no buildables that can be built for the SDKs supported by the run destination iPhone 4.3 Simulator. Make sure you开发者_如何学Pythonr targets all specify SDKs that are supported by this version of Xcode.

I've already played around with various settings in Xcode but nothing helps. Does anyone know how to solve this?

Thanks!

Just a wild guess, but did you have a look in your project settings under Base SDK? Does it say e.g. Latest iOS (iOS 4.3) ?