What do atheists believe happens to you when you die?

Atheism = lack of belief in God.

Atheism still doesn't stop people from believing in other supernatural concepts (concepts other than god). It only means one doesn't believe in god.

So an atheist can believe in reincarnation or belief that one goes to some other plane of existence after death. Many western Buddhists claim to be atheists and while some of them may treat it as philosophy, many of them do believe in reincarnation. The ex-Religion board member "Alexej'" was certainly an atheist who did believe in rebirth. He even differentiated between reincarnation and rebirth and claimed to believe in later but not in the former.

An atheist can also believe that the world is a projection or simulation of a computer system. There are endless possible potential belief systems for atheists.

Though in my personal opinion a considerable number of atheists believe that once we die then we are simply dead as in dead forever and there is no life in any other place. I tend towards this belief. (But I am not an atheist but simply non-religious/apatheist).