Most atheists (Buddhists differ) simply think we stop existing mentally, and decompose physically.

We did not exist for billions of years before we were born, and had no experience of that. After we die, it is a similar situation.

Atheists think this because there is no good evidence to the contrary. Sure, the concept of a heaven is good, however, it would be intellectually dishonest to believe in one if there is no evidence.

Beyond that, the answer is "We do not know."