java - android gradle error with compile - Error:Artifact -

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I have a problem 2 days ago I write an app on Android Developer Tools, compile and install on the phone, All right - all worked well. But now I want to compile it - no, error log

  error: Artifact 'com.android.support: support-annotations20.0.0: support-annotations.jar' not found. Gradle 'test51' failed to refresh the project  

Why ??? I could not change anything, not opened the project notebook is not enabled.

How can I fix this? help please.