Start here. This is the direct spoken answer to practice first.
Why this question matters
Startup failures are common because deploy success only proves that files reached the host. This scenario checks whether you separate deployment transport, application startup, configuration, secrets, runtime, and dependency access.
I would first confirm whether the deployment failed or the app deployed but cannot start. I would check deployment history, startup logs, App Service log stream, recent app setting changes, runtime version, and whether required secrets resolve. If users are affected, I would route traffic back to the last healthy slot or roll back before doing deeper analysis. Then I would reproduce against the staging slot or the same artifact so I can fix the cause, not just restart until it works.