"Life is uncertain, Death is certain" so says the Buddha
We fear unknown and death is like the ultimate unknown. For people who have spiritual faiths, some will fear for all the 'sins' or unwholesome actions that they have done before. They fear the punishment of hell.
For those without any faith, then they may have the extreme side of it. Thinking that death is a certain end of everything, they may try to hold on and to fulfill every single wish for this life... even to the extend of killing unnecessarily.
As practicing Buddhists, we must know that only when we learn and understand death, and the beyond, then only we learn to live with purpose and our fear of death will significantly reduced.
For Tibetans, and the followers of Tibetan Buddhism, we have the great teachings from the worldwide popular book, The Tibetan Book of Death. It has clear explanations of death, the process, the experience and also the guides on how to face death, when the time finally comes...
"Remember the clear light, the pure clear white light from which everything in the universe comes, to which everything in the universe returns; the original nature of your own mind. The natural state of the universe unmanifest. Let go into the clear light, trust it, merge with it. It is your own true nature, it is home."