My published project shows a 404 instead of my site

Check these, in order

1. Is it actually published? An unpublished project has no live URL at all, see "Publishing and unpublishing a project." If you unpublished it (even temporarily, while editing), that alone explains a 404.
2. Are you visiting the exact right URL? A typo in the subdomain, or visiting a similar-looking domain you don't actually have connected, looks identical to a genuine 404.
3. If you're using a custom domain, confirm DNS has fully propagated, see "My custom domain shows a connection/SSL error" for how to check that independently.

If all three check out and you're still seeing a 404 on a genuinely published project at its correct URL, reach out with the project name and the exact URL you're testing.