John of the Cross described two nights — first the night of the senses (when spiritual practice loses its savor) and then the night of the spirit (when one’s very relationship to God becomes opaque). Both are not abandonments by the divine but more accurate approaches to it; the old supports drop because one is being drawn into something more direct.
The phrase has escaped into popular use to describe any severe spiritual crisis. In its technical sense it is specifically a passage — painful, impersonal, purgative, and ultimately preparatory for a deeper union.