The End of The World

Gimme some examples. There's an ancient civilization at the peak of their power, and there's "no way" anything can go wrong.

Centuries later, they're literally extinct.

Emperor has twin sons.

Civilization decides to expand... downwards.

Civilization decides to expand... upwards.

Emperor decides to adopt title "God-Emperor," and something greater takes umbrage.

So this dude tried to conquer heaven and hell and got humbled?

Those were all separate suggestions. Twin sons have a history of ending nations. Expanding downwards invites unsavory things from the deep. Expanding upwards into the heavens invites unsavory things from the darkness between the stars. And finally, pissing off the Gods is rarely a good idea.

The decline of Sumer is a great way to kill a civilization

A couple of generations of the civilization in question decide to cast moderation to the wind and consume far more resources that is sustainable while also wasting the land. Turning a blind eye to such ruinous extravagances seals their doom.

I am writing a scifi epic story where an alien civilization tries to conquer Earth. The only way to subjugate us is to implement a number of ways that they use at the same time. Our civilization is quite stable to be extinct by only a single event. Do not forget that we are more than 8 billion now.

1) They use an asteroid to hit Earth, thus altering the weather globally
2) They use an advanced virus that affects a huge number of the population
3) They use advanced technology and weaponry to destroy our infrastructure and abduct a large part of the non combat population (for slave camps)

After harnessing the atom, particle accelerator
After particle accelerator, god particle
Acter god particle, black hole

"It spans the world from end to end, it drinks the sea without a belch, stretching to the otherside where it is sure to meet itself. Horror of Horrors truly maddening in size, running just prolongs the end, inevitably it strives. Karmatic Armageddon, no religion could foresee. A planet once forsaken. Not even a memory."
-Everything Went Black

Boredom. Boredom led to an immersive and addictive device that allowed them to experience a fantasy world far more realistic and exciting than their own. They exchanged one "reality" for another at the expense of their physical selves.

this reminds me of an old sci-fi game where people mind-uploaded themselves to survive some cataclysm (could be any of the suggestions above).

The question is...
... who is left to maintain the computers once everyone is gone?

The civilization discovered and thrived due to a certain game that was the basis of their culture. Generations pass, and the civilization is at the height of their power. In the end, they find a way to win, thus ascending into a higher reality. All in a single instant, gone.

You all know of this game. That you have just lost it is why you are still trapped in this reality.

Moksha the RPG

I guess this depends on how you define "Ancient." When I read this, I thought of Ray Bradbury's Martian Chronicles. The idea of ancient could be for narrative purposes if the story takes place far enough away from the end times.

Anime cat-girl apocalypse. Humanity meets a race of tabaxi like cat people and within two generations all productive breeding has ceased.

The Last Corporate War was also the first. Of the 388,000 human survivors, the first were dead within a week. Those who didn't understand basic first aid or hygeine perished within 28 days. The few remaining went feral and their species went extinct as we evolved from them, the New Human, Homonovus.

"Now I am become Death, the Destroyer of Worlds" - a line from the Bhagavad Gita, believed to have been uttered by J. Robert Oppenheimer on witnessing the atomic bomb test. They split the atom.