Nothing. Nothing happens to you. You get old or in an accident. Your organs fail or you get a disease. You die and are laid to rest. You body lies in the ground. Then what?
Nothing. That’s it. Perhaps you are found 2,000 years later by a graduate student from the university of Future-Earth, but nothing more. Your memories, your thoughts, your personality, you, in every personal sense, are gone. Maybe your body will inspire a future scientist. Maybe it will aid in anthropology or disease prevention.
It’s a bleak outlook, I know. But that’s all perspective. Everything that made you who you are can live on with other people. That is what a legacy is, though you will not be around to see it. The power of this realistic outlook is that it exemplifies the importance of the life that you are living right now. You only have a few years in the Sun and then you are gone. Isn’t that all the more reason to live you life to the fullest now? Who cares what happens when you die? No one can actually say they know what happens for sure. Science has a good idea, religion has a less reasonable idea (I think), but focusing on the (unlikely) afterlife can keep you from living your life now.
So you will probably succumb to disease or danger, and rot in the ground, consciousness and person-hood dissolved. But that does not mean that life does not have purpose. Your purpose is to live your life.
Right now. Stop reading right now and do something interesting.