No one knows for sure what happens after death, no one has come back after death. Some people have claimed to have after death experience, but there aren't any solid proof to justify their experiences.
As per Hindu Vedas, mainly Garuda Puran, here is what happens after death:
Human life has 3 major cores, one is spiritual core (soul), then mental core(karmic) encapsulating your spiritual core and the final physical core (the physical body).
The physical core is made up from this earth (mud, water, fire, air, etc) and eventually has to go into this earth, be it any religion its true for all of them.
Mental is also known as KARMIC core, which is governed by your mind, your thinking, logic, etc. Whatever deeds you do in this earth good or bad the karmic core keeps record of it. This holds your spiritual core together.
The only thing which doesn't get destroyed even after death is your spiritual core.
So as soon as someone dies; i.e. his heart and brain doesn't function. Only your physical life ends. Your karmic core is still active and so is your spiritual core. Now depending upon nature of death, age, the person's karmic situation at the moment of death. The Karmic core encapsulating the spiritual core can go into anyone's body (animal, insects, person). Generally it takes at least 13 days for karmic and spiritual core to leave this earth.
After 13 days, the karmic core leaves the spirit free to make journey to heaven or hell. It takes around a year for a sour or spirit to make its destination. Sinners are being dragged along the way to hell by pulling their hair. That's why various rituals post death are being performed at least till one year by deceased family members to ease that journey.
There are many more theories regarding life after death. Garuda puran is one of them.
Kumkum Bhagya
