Karma is that hidden ingredient of cosmic life in absence of which human life would not move an inch. Even if we are idling, karma plays its role in the background. For this very reason every single thought precipitates into an act of karma.
Whenever a negative residual balance of karma from previous manifestations undertaken by the spirit, soul atman existing in our heart precipitates, we suffered at the hands of fate. The fructification of karma can occur any given moment of life… now, few days or months later or even after few years or many manifestations henceforth.
A person with a sunny disposition and godly thinking in present life if all of a sudden suffers at the hand of fate; it is negative residual balance of karma from previous lives that has fructified. Human beings in present life time absolutely had no control over this act of karma.
The most human beings could do was neutralize any negative residual balance of karma from previous manifestations by indulging in good karma, positive karma all the time in present life. We can never reverse the doings of past but we can slowly but confidently reduce the effects by engaging in a positive outlook of life in present life.
Positive thinking is the ultimate essence of life following which human beings can not only manifest karma but also destiny forever. The doctrine of karma can be understood in depth by indulging in teachings of spirituality detailed in Bhagavad Gita and also Upanishads (independent spiritual treatises).
Whenever we suffered at the hands of destiny for no fault of ours of present life, it definitely resulted from a negative residual balance of karma from past lives.