"Unable to load nib file: MainMenu"
Just to let you know, I solved the problem by reinstalling the operating system.

This can't be done directly from the app store (because it was not working)

Instead restart the mac and when the grey screen comes press Cmd + R; then connect to the internet and reinstall it. 

I did a backup of my data beforehand but fortunately I didn't need it as all my files were in place.