Sharepoint 2019 web application 503 the service is not available error

Question

I am facing an issue in one of the web applications when I open in browser: 503 the service is unavailable.

I checked app pool, it is in started status. I did iisreset but no change.

Other web applications are working fine on the same server.

I checked in event receiver but there is no clue. I was debugging some custom visual webpart on application with w3wp.exe. Later I did iisreset and suddenly the application was throwing an 503 error.

How can I resolve the issue?

1. I did IIS reset.
2. I re-enter app-pool user account and password.
3. App-pool restarted
4. check event log but there is nothing captured.