What is the power of mind? Can it go beyond our imagination? How does it affect our soul, daily life, health? If a person keep on thinking only one thing again and again, is this a problem related to the mind?
The mind is a manifestation of consciousness. It functions in connection with the intellect, the ego, and our feeling natures.
The mind works primarily through the brain and nervous system, but also there is a strong mind-body connection. Its chief "fuel" or power source is prana (Conscious Cosmic Energy). If you are asking if the mind is powerful, certainly it is — but so are our emotions and the feeling natures of our hearts.
Imagination, or the ability to visualize or "see" things not present before our eyes or other senses is a function of the consciousness or mind also. In its more spiritualized aspect, this is called intuition and can include the "knowing" of things beyond the senses and outside of time and space.
The way we use our minds certainly has a strong effect on our health and daily life. If we think negatively all the time, then negative things will crowd into our lives, drawn by the magnetism of negative thoughts.
The way we use our minds has no effect on our souls which are beyond the body-mind and a part of Divine Omnipresence (God). The soul is the "God-spark" within us and beyond all thoughts, feelings, etc. But our soul’s influence is strong, always for the good; our souls are easily perceived and contacted through meditation.
And finally you ask: "...if a person keep on thinking only one thing again and again, is this a problem related to the mind?" More than likely, it is some sort of obsession related to one’s emotions or feelings. It is probably rooted in a strong desire or attachment of some sort. Attachments can be released gradually and in a healthy way through various meditation techniques.