If it doesn’t load, there should be some kind of error. Either in the browser, or in the network panel (F12) showing (e.g a 404, 500 error code), and/or from running flyctl logs locally from the same folder the fly.toml folder is in. Armed with the error code/message you should see what’s wrong (database timeout, missing file etc).
There was indeed an error. After Googling, I discovered the error was in the fly.toml file. It had the wrong port numbers for some reason. I changed it from 8080 (I think) to 3000 etc.