“It works on my computer but not online”: what's going on

First, look at what the browser says

Open the broken page, right-click anywhere, choose Inspect, then the Console tab. Red lines there are the browser telling you what failed. Copy them word for word. Every fix below gets easier when you can paste the exact message to your AI tool.

It still points at your computer

If the app talks to localhost or 127.0.0.1, it is asking the visitor's own computer, which has none of your app on it. It worked for you because your computer was the one answering.

Search the project for “localhost”. Each one needs to become the app's real online address, or a path without an address at all.

A blank white page

Usually one of two things. The wrong folder went online: the project's source instead of the built dist or build folder. Or the app expects to live at the top of its address and finds itself in a subfolder, so it can't find its own scripts. The console will show red “404” lines for .js files. The blank page guide goes through each cause and its fix.

The home page works, but refreshing another page says “not found”

Single-page apps draw every page from one file, index.html. Clicking around works because the app handles it. A refresh asks the host for an address it has no file for. The host has to be told to answer every address with index.html. Hosts call this a rewrite or a fallback, and the blank page guide shows where to set it on the common ones.

Anything that needs a key has stopped

Payments, email or AI features that worked at home usually read their keys from a .env file that stayed on your computer, which is where it should stay. Add each key to the host as a setting, with exactly the same name.

If the app is a website with no server, a key put there is visible to anyone who looks. Some keys are made to be public. Secret ones need a server, and the guide to API keys tells them apart.

A picture or a page is missing

Your computer may treat Logo.png and logo.png as the same file. Many hosts don't. Check that every name in the code matches the file exactly, capital letters included.

Saved things vanish

If the app saves to a file on your laptop or a database you started there, there is nothing like it online. The app needs a database that lives somewhere permanent, and that means a server.

It needs a server, and the host only hands out files

If nothing above fits and the app signs people in, saves data or sends email, it may need a server, and the place it went can only hand out files. No setting fixes that. It needs a host that runs servers.