My website is down. How do I fix it?
Find which layer failed first: the domain, the hosting account, the server, or the app. What the browser shows tells you where to start. A checklist, in order.
My website says "Not secure". How do I fix it?
The browser can't confirm a private (HTTPS) connection: no certificate, an expired one, one that doesn't cover this address, or insecure content on the page.
500 Internal Server Error after deploying. What does it mean?
Your server code hit an error it didn't handle. The page won't say what; the server logs will. After a deploy it's usually a missing setting, database or file.
502 Bad Gateway: what does it mean, and how do I fix it?
The server in front of your app asked your app for the page and got no proper answer. Usually the app has crashed, never started, or is on the wrong port.
503 Service Unavailable on my site. What's wrong?
The server is reachable but refusing work: it's overloaded, in maintenance, or has no running copy of your app to hand requests to. How to tell which.
My app can't talk to its own backend (CORS error). How do I fix it?
The browser blocked your frontend from reading your backend's reply, because the backend didn't list your site as allowed. The fix goes on the server.
Heroku says "Application Error". How do I fix it?
Heroku's generic page for an app that crashed, didn't start in time, or has nothing running. The code in heroku logs (H10, R10, H14, R14) says which.
My app crashes as soon as it opens. Why?
The crash report names the line. Usually the release build lacks something the test build had: a setting or key, code stripped to save space, or a live backend.
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