Can we speak of a "ontological loneliness"? And what would it be?
Sure can. Forlornness is a central ontological principle in the writings of Sartre, Abandonment for Heidegger.
The central idea is that man finds himself thrown into the universe that is devoid of any inherent or a priori meaning and burden of creating a meaningful life falls on each individual.
Take a look at Sartre's essay, Existentialism is a Humanism; Heidegger's essay, Letter on Humanism.
Or for a general overview, Existentialism: An Introduction, Guide and Assessment by by John Macquarrie is one of the best.