Most atheists 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's more of the same.

Atheists don't believe in an afterlife because it hasn't been sufficiently demonstrated, or demonstrated at all. Sure, the concept of a heaven sounds good, however, it would be intellectually dishonest to believe in one if there is no evidence.

Beyond that, the answer is "We don't know."