This is a quick one; Suppose you have a Free Trial Azure Subscription. On that Subscription, you have deployed a web app on the App Sevice, using the F1: Free App Service Plan, since you are just testing things out. Everything works just fine, until all of a sudden you get a 503 - service unavailable error.
Naturally you find your way to the Azure portal, and start looking, what’s wrong. The problem is, everything seems to be just fine. The app status is Running, no obvious reason to 503s to be seen. You might try to restart the app, with no luck resolving the issue.
Resolution
The official doc on this issue dates back to 2016, but it does offer a still valid clue, what’s going on. Despite the fact, that no portal or console says so, your app has exhausted its resources. Thus, the simple resolution for this issue is to scale the App Service Plan to a paid one, say B1: Shared. Problem solved, your app is once again serving the requests. Just keep in mind, that it is not free anymore per se.