Mac wouldn't start up after I tried to fix a Mac App Store issue I have a MacBook Pro (13 Inch, End 2010) running OS X El Capitan.

I had a bug with the Mac App Store, searched on the Apple forum, and got advice to remove a folder var/folders/zz.  I moved it to the Trash and then the Mac was blocked on the start-up loading bar.
I used Time Machine from Recovery HD to solve my problem, it perfectly reinstalled everything.