One day our project works perfectly and the next day he leaves the red cross. This happens when we took just one year for the same project. What happens is that our certificate has expired, it only has one year validity.
We can see the notice in the "Problem" view, the following error:
Error generating the final archive: Debug Certificate expired on xx / zz / yy hh: mm
solution
To fix it, look where the "debug.keystore" is installed on "Windows / Preferences".
On the right bar select "Android / Build" and watched the path where the keystore is set to "Default debug keystore".
With closed Eclipse, let our users on Windows and then to the ".android" folder. Locate "debug.keystore" and eliminated.
Restarting Eclipse does not generate a new one that will last another year.
But now you want to give a different run an APK (program) in the simulator where it had been installed before the old certificate error. See error: different try reinstalling the application in the emulator Certificate.