Suddenly no electric

You should not just tighten it, take the two apart and clean both the post and clamps. Use a battery post cleaner. I was once fooled by a brand new battery. The posts looked GREAT. I cleaned the clamps, hooked them up, tightened them, and not even a dome light!!!! The posts looked FINE!! but there was just enough oxidation on the posts (or one of) to form an insulating film.