Given your latest comment (which I've taken the liberty of editing into your question), it looks as though the answer is the classic The Hitchhiker's Guide to the Galaxy by Douglas Adams (although it's not an essay or short story). Quoting from here:
In the radio series and the first novel, a group of hyper-intelligent pan-dimensional beings demand to learn the Answer to the Ultimate Question of Life, The Universe, and Everything from the supercomputer, Deep Thought, specially built for this purpose. It takes Deep Thought 7½ million years to compute and check the answer, which turns out to be 42. Deep Thought points out that the answer seems meaningless because the beings who instructed it never actually knew what the Question was.
If the answer wasn't a number, it could also be the short story The Last Question by Isaac Asimov, another classic. Quoting from the Wikipedia summary:
In each of the first six scenes a different character presents the computer with the same question; namely, how the threat to human existence posed by the heat death of the universe can be averted. [...] Humanity asks AC [a super-super-computer] the entropy question one last time, before the last of humanity merges with AC and disappears. AC is still unable to answer, but continues to ponder the question even after space and time cease to exist. Eventually AC discovers the answer, but has nobody to report it to; the universe is already dead.