Thanks for asking question! Could you try restarting your app. On the Azure Portal, on your app’s blade, you have the options to stop or restart your app.
Also, App Service provides an intelligent and interactive experience to help you troubleshoot your app with no configuration required. Access App Service diagnostics, for this navigate to your App Service app. In the left navigation, click on Diagnose and solve problems.
Scaling might be helpful: For more information on scaling, see Scale an app in Azure App Service.
From the metrics menu check to see if you're suffering from resource exhaustion. Try CPU utilization and memory utilization
Refer to Troubleshoot HTTP errors of "502 bad gateway" and "503 service unavailable" in Azure App Service
Let us know if issue remains.