See detail added to my question. I think I've figured it out. (I added a unit-test project to my solution which is being deployed to the web app, and is preventing the runtime from starting up properly).
Azure Web App shows default "Hey, DotNet Developers!" page after deploying new version
I've been developing an Azure Web App website for a couple of months now. Today I deployed a new version (which I do almost every day), and suddenly my web app is gone.
When I go to the URL (https://<my-website-name>.azurewebsites.net) it just shows the default "Hey, DotNet Developers!" page. If I ask for a static file (e.g. favicon.ico) it just gives me a 404 Not Found.
I don't deploy directly - it automatically deploys the website when I check source-code changes into GitHub. Looking at the deployment logs in Deployment Center, it says that it has successfully completed the deployment.
When I look at the Log Stream for the app, it says that it's "initialized successfully and is ready to serve requests".
If I go to Advanced Tools and look at the Site WWWroot then I can see a bunch of files in there that have the expected date and time.
And yet, no website! What happened?!
--- UPDATE ---
Well, I think I know what caused this. It wasn't obvious from the Log Stream, but when I used the Advanced Tools to log into a Bash shell and looked at the logs in the /home/logfiles folder, I saw this line:
DotNet Runtime 5.0WARNING: Expected to find only one file with extension '.runtimeconfig.json' but found 2
This was only a warning, but I think what's happening is that it can't decide which DLL to run, and is either running the wrong one or possibly neither.
And the reason there are two is because my latest check-in added a unit-test project which I can see is being deployed along with the other projects, and which does indeed have a ...runtimeconfig.json file.
Now I'll just have to figure out why it's doing that :-)