Classes disappear from packages

Hi to all!

I’m new to this forum, and the title of this thread could be misleading. The problem is this: I downloaded eclipse Luna few days ago, and started working with it on a project I had in one of my repositories, by importing it (without copying the files in the workspace). Then, since Luna didn’t allow me to use some plugins (as fatjar, for example). Moreover, the Luna release started behaving really strange. Without any change, it started throwing the error “Unable to locate or load the main class”, but only with imported project. Creating a new one inside didn’t get the same effect, and worked properly. I decided to switch back to Kepler, that was the last version I used without such problem. I then erased Luna and downloaded Kepler. Anything changed. Every time the same error. Today (all these yesterday), I opened Kepler and the package explorer was showing me the project but with the main common package empty.

I tried different solution by searching on google, but no one worked.
Where’s the problem?

Thank you in advance