The only thing which disconnects one from others is a one's ego. If you ask what sfeera is related to such state - it's malhut. But it doesn't really matter in this case. Nothing wrong with the person really, since everybody's nature is a desire to receive (ego).
All people are just one. This one called Adam Arishon. He just feel himself like one and other 7 billions. He feels this way because of ego.
How to solve this problem? Just start to receive for God's sake. Only for Him (since He desires to give) and nothing for yourself. To get knowledge of such state person has to practice on others (love your neighbor as yourself)..
This is very rough explanation, you'll find better from true kabbalists, which now revealed and available for everybody.

You might find some answers in Rabbi Abner Weiss's book "Connecting To God: Ancient Kabbalah and Modern Psychology"

The paths between the spheres are still connected. They haven't short-circuited. All the sefirot--with the exception of Keter--are involved to some extent.

Problems arise when some sephirot become overdeveloped and others are atrophied as a result of past experiences (usually in childhood). When sephirot are out of balance, they become encrusted by kelipot (husks) and shadow characteristics and behaviors emerge.

Weiss regards inability to carry through on plans as an imbalance in the energies of the Chokhmah-Binah-Da'at (CHaBaD) upper triad.

According to Weiss:
CHaGaT (Chesed-Gevurah-Tif'eret) is the triad of feeling
and NeHiY (Netzach-Hod-Yesod) involves the sefirot of relating to others.
Detatchment and disconnection often result when the left side sefirot (Binah, Gevurah and Hod) are overdeveloped.

Weiss' approach to repairing the imbalances involves multiple theapeutic strategies and spiritual practices (including guided visualizations) based on the individual's needs. Check out his book for more. I borrowed it from the San Fran public library:

www.connectingtogod.com/

Joy, I would just like to suggest one thing to keep in mind: the map is not the territory.

The functioning of the mind (what you are describing is known as "dissociation") can be mapped in a wide variety of ways. The ToL is one of those maps, and IMO, a very useful one at that.

But I would also suggest that it is important to look at the person, not the map. That is, what does the person need to become more associative rather than how the ToL functions. I would contend that failure to do this among some clinical therapists--who try to fit their clients into their favorite healing modality rather than having access to a variety of healing modalities and selecting the one or ones that will fit the client's needs--is a primary cause of failure in establishing positive change.

Be that as it may, a common look at the mental aspects of the ToL, going from top to bottom, is as follows:

1=Yechidah (True self; Transcendental ego; Freud's "superego")
2=Chiah (True Will; Our creative and inquisitive inner impulse aligned with the Will of God)
3=Neschamah (Intution)
4= Memory
5=Will (that can be moved by superficial and transitory desires)
6=Imagination
7=Desire
8=Reason
9=Nephesch (The "lower" self; Freud's "Id")
10=Guph (The physical body and the way it influences us)

Using this model, you would have to look at the person with the problem and discover what is the source of where the problem lies. That is, what could cause dissociation. If the problem was physical abuse, the cause might be dwelling in the 10th Sephira (escaping from the physical). If the problem is ennui, the cause might be in the 7th (no desire to move on and live). If the problem is caused by a dissipated lifestyle, the cause might be in the 8th Sephira (No logical reason to associate).

Thus, there is no single answer to your question. Dissociation is not a cause but a symptom, and you have not given the cause. Not only that, but unless you are a trained professional, it is possible that your analysis of the cause might be in error or only a superficial cause and not the real cause. That's why professional don't diagnose in absentia.

But let's say you have found which Sephirah is the source of the dissociation. IME a great technique for resolving this is working with the Tarot major arcana. Now, it can be pointed out that historically the Tarot and Kabalah developed separately and have only been linked over the past few hundred years. IMO that's irrelevant. Both the Tarot and ToL are maps, not the reality. Using the two together is simply an overlay, not the law or life.

The ToL map, then, has "cities" (Sephiroth) and roads (paths between the Sephiroth). The major arcana of the Tarot function as the names of the roads.

So let's say, for example, that the cause of the problem is physical abuse. On the over all map, the "opposing" city from a visual perspective would be the True Self at the top of the tree. The road to that has the names Universe, Temperance, and High Priestess. Finding a Tarot that deeply appeals to your unconscious and spending time meditating on those cards can help take a person to their True Self.

Or perhaps the cause is ennui, a lack of desire (7th Sephira) to move ahead. The opposing "city" is the 8th Sephira because such a person needs a reason to recreate that desire for life. The "road" is named Tower, and meditation on that card can help a person discover reasons to associate.

Unfortunately, this is a simplification because we do not have a specific client. Sometimes, taking the most direct route on the map won't solve the problem. Instead, you have to take the scenic route. For example, perhaps the client has led a life of dissapation and has no reason to move on. The goal state is to have a desire to move forward, but the person is so "stuck" in reason that something more is needed to achieve that desire for life. That "something" could be imagination for what could be. In which case the road would be named Devil and Death to be reborn. Or perhaps the person needs to re-establish their imagination(6th Sephira) so they can remember (4th Sephra) what they enjoyed in the past and rekindle a Desire (7th Sephira) to live again. In this case the names of the road would be Devil, Hermit, and the Wheel of Fortune.

I hope this gives you a bit of an idea of how this process can work. Is what I've described the only way and only interpretation? Absolutely not! I'm sure there are many others. However, they are all only maps. Each person has their own map and IMO it is important to use your client's map rather than forcing them into following yours.