How to fix HTTP 404 on Github Pages?

In my case, I had folders whose names started with _ (like _css and _js), which GH Pages ignores as per Jekyll processing rules. If you don't use Jekyll, the workaround is to place a file named .nojekyll in the root directory. Otherwise, you can remove the underscores from these folders

I did all the tricks on my repo to fix page 404 on Github Page (https://eq19.github.io/) but it kept 404'ing.

Finaly found that my browser hardly keep the 10 minutes cache before it up on the web.

Just add /index.html into the end of URL then it showed up and solved the case.

https://username.github.io/{repoName}/index.html

I had just one commit with all my files. I pushed an empty commit, refreshed the page and it worked.

git commit --allow-empty -m "Trigger rebuild"
git push

If this doesn't work, as @Hendrikto pointed out in the comments, check out the Github status page and make sure GitHub Pages are operational.