A published app will not open
The usual reasons a live app does not load, in the order worth checking.
Preview is your working copy; the live address serves the version you last published. When the app works in Preview but the address does not open or shows an error, the cause is almost always one of the reasons below. Check them in this order — each has a clear fix.
The subdomain changed
This is the most common cause. Choosing a new subdomain for the app replaces the old one, and the old address stops opening immediately; there is no redirect to the new one. Open the app, select Open Session, then Publish in the bar above the app, and use the address shown in App Link. Send that address to anyone who has the old one.
The address is mistyped, or the app was deleted
Copy the address rather than retyping it. In the publish panel, App Link shows the exact address, with a copy control beside it. An address that is close but not exact, and the address of an app that has been deleted, both show only an App not found message in the browser window.
The app was never published
A reserved address is not live until you publish. If the panel's App Link is labelled Not published yet and says This link opens once you publish the app, select Deploy. Visitors who open the address before then see a page headed This app isn't live yet. If you published earlier, remember that the live app is the version from that publish — changes made since appear only in Preview until you deploy again.
A build failed, so the old version is still live
If the last publish failed, your change is not on the live address; the version published before it stays live, and the page looks exactly as it did before. The publish panel shows The build failed. Ask the agent to fix the problem in the session, then select Deploy again. After a successful publish the panel shows Deployed with the date and time. When a build does not work covers the build side in more detail.
A custom domain is not verified yet
If you connected a domain you own, it starts serving only after verification. Open Settings → Publish and look at the Verify your domain card. If the domain is not marked Verified, add the value shown there to your domain's records where you manage the domain, then select Verify. Until then, a visitor to your domain gets a browser warning that the connection is not secure rather than a page.
A domain that is claimed and never verified is released after seven days and can then be taken by someone else; add it again and verify it. Your AppHandle address keeps working throughout. See Connect your own domain.
The app is paused
When an account has spent the month's included usage and its balance is empty, apps that use metered services stop serving. Visitors see a page headed This app is paused, saying the app is unavailable right now, and adding credits makes it resume serving automatically. The allowance renewing at the start of the next month does the same. Nothing is lost and nothing needs rebuilding. When the allowance runs out explains what else stops.
The app is private
If Access is set to Private, only you and the people you invite can view the app; everyone else is asked to sign in before it opens. If the app is meant for anyone with the link, switch it to Public. See Choose who can open your app.
Someone signed in and is still refused
An AppHandle account on its own does not open a private app. A person who signs in without being added to the app is refused. Add them under Settings, then Team. Share your app shows the steps.
If the address still does not open after these checks, copy the exact address and Get help.