This part of the Discipline of Solitude is made separate, as it can be harder to understand for many people, and if you are still early in your journey, you may want to skip over it.
The "Dark Night of the Soul" is a condition that has long been described by men seeking a deeper relationship with God. It is NOT something "Bad" or "Destructive", and not something to be avoided. When we first experience it, it may seem to be something to be avoided, but is actually God providing an opportunity for special growth.
The Dark Night of the Soul is a period when, particularly when practicing Solitude, God just withdraws some of the normal joy of the experience. We may feel "dry", or alone, or even maybe a bit of lostness. We stop to search for God, and for the moment can not find him.
Our temptation in times like this is to back out, and avoid them, but all this does is plateau your spiritual growth and leave you actually stagnate for a while, until God renews the offering again.
This is also not a condition caused by sin (though sin may produce similar symptoms). The answer for the one i that case is just to repent and experience the joy and refreshment that comes from repentance.
The answer for how to handle the Dark Night of the Soul, can be found in the scriptures:
Who among you fears the LORD
and obeys the word of his servant?
Let him who walks in the dark,
who has no light,
trust in the name of the LORD
and rely on his God.
--- Isaiah 50:10
So, the answer when we come to this sort of time, is to just deepen our trust of God, and let him see us through this.
The Dark Night of the Soul can in one aspect be seen as an echo of the conversion experience, with a deeping of convictions and dependence. Just like, before a Sinner can become saved, they need to reach a point of "exasperation" where they see that they can not save themselves, but must depend on God, the Disciple who is trying to grow in his spirituality, will reach a point (perhaps several times) where God just moves his heart to see that they aren't really depending on God, but still on themselves, and God withdraws the satisfaction that comes from their walk, and hopefully draws them to a state where they turn themselves over to be under the control of God in new ways, and let them grow in much deeper ways.